*If* you're fortunate enough to come upon 8-foot boards, then use them. Cutting one 8-foot board in half will give you two 4-foot ends of a raised bed. I have some smaller square "frames" that we stacked up for a potato tower but are right now acting as small raised beds because of the season.
Here's a thread with lots of suggestions for low-budget gardeners, including where to find materials:
Most of the veggie gardening here has been done on a scavenge/low-budget basis. This year, we've actually purchased some plant starts b/c our windows for seed starting were trashed by other circumstances. There have been many discussions, and some photos, here of raised beds, so there will be help in that department, too.
Sunset Zone 17, USDA Zone 9