Depending on the module. A lot of class 4B modules are 16t or less, but there are a few which are heavier: a 4A Prismatic or 4H SRV bay are 20t each, and a 4E HRP can get up to 22.4t with engineering. Alternatively you could fit a class 3B G5-reinforced limpet controller in the slot and get up to 32t despite the module being 1 class undersized...Therefore saying a slot is a size 4 means that you can fit something that has a mass of or capacity to hold 16t.