We replaced our foreberth foam last year, and were confronted by too
many choices. Go to a good foam or upholstry shop (furniture, not
boat) and ask. They'll usually lay some foam samples on a table and
let you sit/lay/jump on them. You can then choose either a single 4"
foam, or you can have them laminate foams together -- soft on top, firm
beneath -- for a custom feel.

Some people swear by natural latex, but it's no more resistant to
mildew, and costs MUCH more than poly.

Sleep tight!