Can I use 1x4 boards for raised platform support?
Joined perfectly, 1x4 boards would work. However, you'll get more nailing (/screwing) surface, possibly lower cost, and only slightly greater weight from 2x4s.

You mention center support beams... I'm not really clear on how you plan on putting those in, but you probably don't need them with 2x4s.

I'd suggest at least 1/2" plywood on top.