Hi smmimp13,
With your higher weight I think that you would be much more likely to “go through” most softer 4 lb memory foam toppers and I would focus on higher density memory foam and possibly firmer memory foam as well if you are committed to using a memory foam topper (even though that may seem counter intuitive to you). 5" of memory foam on top of a mattress (with both of your toppers together) along with any softer layers in the mattress underneath it could also be very risky in terms of alignment and this may be part of the reason for what you are experiencing.
It may also be connected to the mattress you are using under the toppers because if there are any soft spots or impressions in the mattress underneath your topper then a topper will just “follow” any impressions or soft spots underneath it.
I would also check your foundation to make sure that it also isn’t sagging under your mattress topper combination (you can test this by putting your mattress/topper combination on the floor to see if it makes a difference).
Knowing the specifics of the mattress you are using under the topper would also be helpful as well.
If I had to guess based on the very limited information you’ve provided I would guess that your current issues may be more connected to too much softness/thickness in the upper layers of your “sleeping system” than too much firmness.
Post #2 here and the topper guidelines it links to can help you use your actual sleeping experience on your mattress as a reference point and guideline for choosing the type, thickness, and firmness for a topper that has the best chance of success but again if I had to guess I would guess that your comfort layers/toppers are probably too thick/soft for your body type.
Phoenix