What's an easy way to build a level 6'x4' platform over uneven indoor floors?
This is small enough that you can simply build a deck out of 2x4 with a 1/2" plywood top (or thicker, depending on flex tolerance). You're essentially building a typical interior wall structure, but smaller.


Lay out a simple rectangle with joists at 16" centers (across the short dimension). Fasten it together with 16d nails or 3" gold construction screws. Slightly shorter fasteners could be used for the plywood.
Using a 6' level (or a shorter level with a straightedge extension), find the difference from the highest point to the other three corners, and cut legs or simple spacer blocks to level up the platform. 


Large felt pads under each corner would make it quieter as you step up and move around.