We are having new countertops manufactured for our kitchen. The one countertop is "L" shaped, with one side of the "L", which is 4 feet long, supported by the cabinets underneath. The other side of the "L" (27W x 68L) is unsupported. Let me mention that the old countertop, which sagged, was supported by a wall beam horizontal stud on the back edge underneath and two 4x4 decorative posts near the end of the peninsula. My question is how to support the new peninsula? We are having the countertop made from custom laminate. A friend of mine suggest a boxed unit (10W x 58L) under the middle made from 1x4 oak, screwed into the wall studs and then supported on the peninsula end by either one or two new posts. We eat at the peninsula quite often with our two young kids so we are concerned about keeping it as open underneath for seating. Any help and suggestions would be appreciated. Thanks.

This peninsula your speaking of. Is it agains a wall or are people going to be able to sit on both sides of it?
If so the box idea with a few legs would be the best bet. I would however build this beam out of plywood or a microlam beam then cover it with oak or what ever cabinet grade covering you need.
Simply building a box with traditional lumber will also sag over time. While the many layers of plywood because of its construction is very strong and will not sag.