As long as you already have it, have you tried it folded over to give you 2" on one side, just to see how that works? That is one of the nice things about using multiple thinner layers, is that you can configure them to try other options without having to buy a lot of thick, expensive foam. Always a good idea, while you have a sheet of foam in your possession, to try any experiments you can with it, since you may wish you had later.
