Cut two of your 4-foot strips to 3-feet-2-inches with a saw, then cut the remaining two to 3-feet 1/2-inch. Lay these out so they form a square with the narrow sides facing up and place a small amount of wood glue where they connect. Nail together with finish nails and let the glue dry. This is your table's frame.
Cut your wood planks into eight 3-foot-6-inch lengths. Spread a light coat of wood glue over the top edges of your frame and lay the planks so there is approximately 2 inches of overhang on each side. Nail the boards into place with finish nails, which have hardly any head so they will not be obtrusive on the finished tabletop.
Cut four pieces of 2 by 2-inch lumber to the height you want for your table, minus 3/4 inches. Flip the table over after the glue is dry and place one leg inside each corner of the frame. Place a little wood glue on each connecting surface--including where the top of the leg meets the underside of the table--and secure them with flat-head nails from both sides.
Flip the table back upright and apply a clear wood sealant (varnish) to all surfaces using a good quality paintbrush or sponge brush. If you prefer to stain the wood, apply the stain first, then the sealant.