Taking my own post from LR forums, see below. Additionally there are significant pricing differences between newer 4's and older 3's.
REF:
http://landroverforums.com/forum/lr3-28/lr3-vs-lr4-76448/
EDITED TO REFLECT ERRORS IN ORIGINAL POST (thanks expo people for bringing some up in posts, Umbertob and mpinco)
"This will be a long one, so there will be a
TLDR at the end . I have driven both, but own a 3, so accept my bias with a grain of salt. I will say the 4 is one of the smoothest driving trucks I have ever experienced, and I've driven most of the Ford F-series from the 150's to 550's.
Interior: x2 what earlier poster said about interior differences. IMO the 3's interior versus the 4 is still very nice, but you feel more like you're driving an overland-able truck instead of a mall crawler. Also the design update removed the handles on the front headrests, something which I aesthetically enjoy AND almost every passenger I've had has mentioned they liked when I'm going over rough ground (so they can hold on). The controls are simpler in a 3, no frills, and I prefer the muscle memory of the auto shifter in the 3 (NOTE: the discovery 4 retained the shifter until 2014 when they removed it) to the stupid dials and paddle of the newer 4's (2014MY-). With the 3 you get way more physical buttons and I have always loved the layout, everything is more or less logical to me. Every once in a while I visit the sales staff at my dealership and during a few tours of the 4 I have had to fish for what buttons or settings I want. I'm sure its something you'd get accustomed to if you owned one, but my that's my take. On the 3 only the drivers window is one-touch, something I constantly find annoying. The Discovery 4 has a smaller/skinnier steering wheel in the newer 2014- models. I do not recall when the Discovery 3 started getting heated steering wheels added as part of the cold climate package, but the 06 did not have it.
Exterior: I love the front end of the discovery 4 but that's about it.
They're so similar exterior wise that it mostly comes down to the grill, fender flares (mentioned by HOUM_WA), headlights, and taillights. I like the older look with the more overt amber indicators of the 3, but that's a personal preference. The 3's have great headlights with the xenon bulbs anyway, and I can adjust the beams in my driveway with a screwdriver and they still look very modern. Sometime after 2007-08 I believe trucks started getting auto-leveling headlights which you cannot adjust with a screwdriver... I hate the running lights constantly on but I think you can disable that feature somehow on the 4's. The 3's are manually operated, no LED running lights. 3's can also be equipped with the LED taillights that the 4 has, but you will need to add ballast to the wiring (check the how-to thread someone recently did on the forums here). Functionality-wise the 4 made roof rails an extra option too so it will be annoying if you get one without rails and then want a roof rack. 3's come with rails standard. Easy upgradeability. Discovery 4 added an air intake on the drivers side for a total of TWO air intakes. 3 has a right-side air intake only, which means you can put a snorkel/RAI on it if you REALLY want to look super cool :snorkel:.
Engine wise, while I do appreciate what they have done for fuel economy (this applies only to the SCV6) and instant power with the 5.0 V8 and the 3.0 SCV6, the 4.4 seems like the "bulletproof" (and I use that word loosely ) option since they're powerful enough, simpler to work on, have better parts availability, and are more widely used. Even the TDV6 guys (the engine you would want if you could get one) have mentioned a few times that they would appreciate the extra muscle of the V8. I have seen a few posts about guys with the 5.0 having problems that others could not figure out because we're used to working with the 4.4 so safety in numbers?... For now the most info is out there for the TDV6 and the 4.4L as these trucks are getting older and needing some maintenance. Within a few years the 5.0 may start to be in the same boat.
Discovery 3
2005 4.0V6 or 4.4V8
2006-2009 4.4V8
Discovery 4
2010-2013 5.0V8
2014 3.0SCV6
Transmission: the 4's I've driven have the 8-speed which while being smooth, seems like way too much of a complexity gamble for a vehicle which ostensibly should be up for some heavy abuse. The 6 speed is the sweet spot for me, especially since its a ZF tranny which will treat you right if you flush fluids every once in a while (75k rather than the 150k LR suggests)(I will admit that they do have problems on some trucks...). Unfortunately the market is favoring smoothness (more gears/changes) over robustness (less gear changes, easier to service/repair, longer lifespan). The disco sport AFAIK has a 9-speed...
Discovery 3
2005-2009 8-speed ZF
Discovery 4
2010-2013 6-speed ZF
2014- 8-speed
Wheels: I thank my lucky stars every day that I have 18's and not 19's. I have heard anecdotally that some guys have forced their dealers to trade their Disco 4 19's for OEM Disco 3 18's (which should fit with minor spacers, or minor grinding to brake caliper housing-read the overland journal LR4 thread), I feel bad that so many 4 owners are stuck with a ****ty wheel/tire choice as stock. The compmotive 18" are excellent aftermarket options for a disco 4 but are $$$, which a 3 owner would otherwise not have to spend.
Differentials: the 4 has had all the updates to the known issues the 3 had with differentials, which usually came down to a better breather and the fact that early models had the insides of the diff painted? (never confirmed this). Unfortunately a lot more things are options on the 4's so its now a ~$1500 optional extra to have the HD package versus a ~$600 option a few years ago for a 3. Expect to swap in a new diff or two, and make sure you update the breather for the front diff (its ridiculously easy, you can do it standing up with scissors). The locking diff models for the 3's are notoriously hard to find for sale BUT now that ARB has air lockers, that is not an issue.
TLDR:
Interior: Discovery 3=more rugged but nice, front seats have grab handles; Discovery 4= way more luxurious, way more buttons, later years deleted the shifter knob in favor of a dial/paddle shifters.
Exterior: Discovery 4=nicer headlights, more LED's, different grill, paint matched fender flares, different tail lights, no roof rails as stock, TWO air intakes. Discovery 4 front end was refreshed for the 2014MY. Discovery 3 simpler lines, simpler exterior, overall identical shape, YES roof rails stock, early models were available with halogen or Xenon lights, air intake only on right side of NAS vehicles.
Engine: Discovery 4=5.0L V8 (from 2010-2013) or 3.0L SCV6 (standard starting 2014MY); both are nice. Discovery 3=4.4L V8, or (in very rare cases) a 4.0L V6; 4.4 is proven, simple, good compromise
Tranny: Discovery 4=6speed or 8speed, Discovery 3=6speed, your choice really.
ADDED: Compressor: The Discovery 4's have the AMK compressor for the EAS now instead of the Hitachi's that the Discovery 3 came with. The AMK can be swapped to the Discovery 3 however, but it should be said that the 4's in theory have better compressor reliability.
Wheels: Discovery 4=19" wheels only, this sucks (can buy aftermarket 18"s, but this=$$$). Discovery 3=18" wheels stock, great size for tire selection, still look good.
Differentials: Both trucks were available with HD package as an option which added an e-locker @ rear. Discovery 4=most problems fixed, more $$$ for the HD than before (2014 and onward years made the low-range box as part of HD package, so base trucks do not have low range as standard anymore). Discovery 3=expect some problems, still okay though, ARB makes lockers (air/mechanical, not electronic so does not interface with LR computer systems) if you can't find HD package models."
Pricing: Varies widely, but generally expect to get more bang for your buck with older vehicles. Newer ones may have warranties still, and obviously lower mileage, BUT there are a quite a few Discoveries still around with one owner that has taken great care of it and has perfect service records. A lot of original owners sell them either before the 75k maintenace period, or around 120k and onwards. They usually do not have warranties beyond 50-60k unless they had an extended one.
