When I built my first bed system, I was trying to maximize under-platform height to fit my storage bins, so I used 4 steel legs on ~17" spacing. Notice there's no cross-bracing, I couldn't spare the extra height, the bins just barely fit. With this setup, the entire forward edge of the platform was also supported on a box.
Turns out this was WAY overkill. On my new setup, I removed the box and moved the legs to the corners and I now have an unsupported span of about 50". I don't have a great picture of the current setup, but you can sort of see here how there are no legs in the middle span area. With cheap Home Depot 3/4" CDX plywood, this works fine for at least myself or my wife+kid. Haven't tried me+wife on this setup yet, but I suspect it would be ok but bow slightly...