Like they said...
4' wide is easy to do with plywood or particle board. For a table top, I'd actually suggest plywood rather than "real" wood - it's a lot more stable. Avoid particle for anything that might get wet.
Stability... wood "moves". That is, it expands and contracts as the humidity changes. This happens mostly over the width, and slightly (and ignorably) over the length and thickness. But the width is a problem. Say you glued up a 5' wide panel (the bar clamp and biscuit idea above), then screwed a piece of wood perpendicular (say, for legs). The table top will contract when it gets dry, but the legs won't - and you'll get cracks. That doesn't happen with plywood.
If you want to do the 5' wide wood, the terms to look for are "edge joined". It's easy to do with proper tools (tablesaw to start with), and a pain in the neck to make look good if all you have is a circular saw. I'd be happy to offer advice, but if you haven't done woodworking it's probably not worth the pain.
Having said that... plywood. Take a couple of sheets of 4x8 ply (standard size) and laminate them. The "top" will be a 4x5 piece and a 1x5 piece - and then the same on the bottom, but with the joint opposite. Use a lot of glue and some cinderblocks and it should come out fine. If you start with 1/2" ply, you'll end up with a 1" thick surface.