Hi lilac2,
I’m not sure where you read this but it’s certainly not something that I would write 
Memory foam is certainly a good pressure relieving material in the right thickness, softness/density, and depending on the other layers that are part of the sleeping system but there are many other materials that also do a great job at relieving pressure.
The 5 lb Aerus will tend to feel firmer for most people.
This would really be a preference choice and would also depend on the other specifics of the topper and the mattress underneath it. 4 lb memory foams that are made by different manufacturers can have very different properties even though they have the same density (see post #9 here and post #8 here). It could be “better” for some people and “worse” for others.
If you are looking for additional softness and pressure relief for a mattress that is too firm then it would generally be much more effective to add a topper on top of the mattress than adding something underneath it. The upper layers of a sleeping system will have much more effect on pressure relief and comfort than the bottom layers.
Phoenix