Rolling platform to support 4000lb
I would frame it like a heavy-duty wall. Each side beam needs to support 2000 lbs. I calculate that three 2 x 6's are required on each side.





16 inch spacing of joists should be sufficient, use 12" to be more conservative. 

And if you want it to be really strong, put plywood on both sides.

And I agree with Ecnerwal's comment, this thing will be a dangerous beast.