First, if you haven't done so already, I would HIGHLY recommend you take the MSF Basic course.
Most of the major players in the 650 thumper category (KLR, F650/G650, DR650) make great commuters. The DR is perhaps the most dirt worthy and the KLR and F/G650 are probably a bit better on the highway. But it is really splitting hairs. The KLR and BMW come stock with good fuel range and decent seats. Only the BMW has FI. For $4k, I don't think you will find the newer G650GS. You're probably going to be looking at an F650 prior to 2005 or so. (For reasons known only to BMW, they changed the name of the F650 single to the G650 when the F650 twin came out. And of course the F650 twin is 800 cc. I dunno - ask BMW!)
But if your idea of "offroad" is gravel and forest roads, I would also look into a Suzuki DL650 (aka V-Strom) or even a Kawasaki Versys. These are twin cylinder 650s that would be much more highway friendly and much better 2 up. Both are also fuel injected.
But here's the good news - all the bikes listed above are nothing short of GREAT. Find the one you like (that's been well maintained) and get it. You will find lots of people who have bought one and feel the need the criticize the others. But the fact is they are all great bikes in their own way and it is hard to go wrong if you buy a well taken care of example. You can always sell it after a season for very little loss if you find you want something else (which very well may happen).