Hi saraturner,
There are 5 lb gel memory foam toppers available yes but they tend to be less common than 4 lb gel memory foam. There is one here for example.
I would also be very cautious about using ILD/IFD when you are comparing the firmness/softness of different memory foam materials because it isn’t a very accurate measurement of firmness when it comes to memory foam (see post #2 here).
The most reliable way to know the relative firmness (or other properties) of a particular memory foam topper would be a conversation with a knowledgeable retailer or manufacturer that sells it.
Phoenix