Hi WebbyR,
A topper can add to the durability of the layers below it yes but adding a topper will also make the comfort layers thicker and softer which could be risky in terms of alignment if your mattress is already a good match in terms of PPP. The top 4" of your mattress is already latex so it’s already a durable material so I would think the risk of a topper would outweigh its benefits.
A mattress pad (with padding in it) will change the feel of the mattress vs a mattress protector without any padding. If the mattress is already a good match for you I would go with a protector. You can see some of the pros and cons of the different types of mattress protectors in post #89 here. A 2" topper can have a significant effect on the feel and performance of the mattress and could also increase the sleeping temperature (you would be sinking into more soft foam which would be more insulating).
If you are comfortable with the mattress already I wouldn’t add a topper … especially to a mattress that already had latex in the top layers.
Phoenix