There are many factors to consider when evaluating memory foam toppers. Density is certainly one of the important factors, although it mainly relates to the weight of the foam per cubic foot and its durability rather than firmness or hardness.
For comparing firmness, see if they can provide you some info on the ILD or Initial Load Deflection (also known is IFD / Initial Force Deflection), which is a quantitative measure of the foam’s firmness or softness. This important information will help you determine whether there is any difference in the four different thicknesses of foam toppers you asked about.
Next, you need to consider the firmness of the underlying mattress. For example, if the underlying mattress is extra firm, and you put a thin topper on it that is very soft, you will simply sink right through the topper onto the firm surface of the underlying mattress. The net effect of this might be a pretty firm feeling. By contrast, with this same extra firm mattress, you may find a softer overall effect can be created by using a topper that is slightly firmer, but is thick enough that you don’t sink all the way through it. Industry pros usually recommends 2 inches minimum in order to make a real difference in mattress feel.