Hoffa, I'm not sure of the "brand names" but we bought 6" memory foam, the kind advertised that you can jump up and down and not spill your wine (as if I leave my wine glass on my bed). We cut it for size for the v-berth, and had it covered with mattress material to make it look good. I personally think it's fine without being covered, but my "admiral" insisted. Two pieces, as we have storage underneath and need ease of access. It has been fantastic. I don't know that 4" wouldn't really work, but it really sleeps fantastic at 6". Good luck in your endeavors, we're really pleased with ours.
Mike and Joyce, S/Y "Tattoo"
"There is nothing- absolutely nothing-
half so much worth doing
as simply messing about in boats."
The Wind in the Willows