Hi Heatherly30,
The density of memory foam is more related to its durability and not necessarily to its softness or “feel” because any density of memory foam can be made softer or firmer although in general terms it’s often true that higher density memory foam will “feel” a little firmer, be more temperature sensitive (have a wider range from softer response to firmer response), and have a slower recovery time than lower density memory foam. You can read more about the different properties that can be chemically formulated into different types of memory foam in post #9 here.
5 lb or higher memory foam is more durable than lower density foam and I would generally tend to minimize the use of 4 lb or lower memory foam in a mattress for people who are in a weight range above 200 lbs or more because of its lower durability but in a topper it can still make a good choice because it can be replaced when it softens or breaks down without having to replace the whole mattress and many people prefer the “feel” of some 4 lb memory foams. Memory foams lower than 4 lbs would be even less durable and have less of the “memory foam” properties that is the reason for memory foam in the first place so I would tend to avoid them unless durability isn’t an issue.
When you are buying a memory foam topper that you haven’t tested in person it would be important to talk with the online store or supplier to get a good sense of the softness and response of the memory foam you are considering and then use the density of the memory foam to compare them in terms of quality/durability and cost (higher density memory foam is more costly than lower density memory foam or the same type).
The topper guidelines in post #2 here and in post #8 here may also be helpful in choosing a topper.
Phoenix