Hi Raynan,
It would be helpful if you could provide some additional information about your body types and sleeping positions and some additional information about any specific symptoms you are having when you sleep on your mattress.
Unfortunately you are the only one that can feel what you feel on a mattress/topper combination and there are too many unknowns, variables, and personal preferences involved for anyone to recommend a specific topper for someone else based on specs (either yours or a mattress) or “theory at a distance” with any certainty (see mattress firmness/comfort levels in post #2 here) so it may take some experimentation and trial and error to find the type, thickness, and firmness for a topper (or other additional layers) that works best for you.
While each person is different and unique so what may work well for one person may not work well at all for someone else … in very general or “average” terms I would tend to agree with both of these comments. There is more about the effect of thinner layers or mattresses (which tend to be firmer) in post #14 here and with only a thinner comfort layer on top of a firm/thin support core you would probably be feeling too much of the firmness of the latex support core “through” the 2" topper.
This is quite “possibly” true as well but it would depend on your body type and on the specifics of the topper. The only way to know for certain would be to sleep on it in person. I would tend to avoid 4" memory foam toppers unless you are very large or very heavy because it can be a little more risky in terms of maintaining good alignment over the course of the night because memory foam gets softer in response to temperature, humidity, and the length of time it is continuously compressed so you may start off the night in good alignment before it has softened but you may find that the heavier parts of your body sink down too far over the course of the night and you are out of alignment when you wake up in the morning. Thicker layers of memory foam can also sleep warmer than thinner layers because they are more insulating and they may also be more motion restricting as well.
There is more about the more common symptoms that people may experience on a mattress and some of the most likely reasons for them in post #2 here.
Post #2 here and post #4 here has more information about primary or “deep” support and secondary or “surface” support and their relationship to each other and to firmness and pressure relief and the “roles” of different layers in a mattress that may be helpful in clarifying the different types of “support” and “pressure relief” which are the two main functions of every successful sleeping system.
Post #2 here and the topper guidelines it links to also has more information that can help you use your sleeping experience on a mattress as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the best chance of success and also includes a link to some of the better topper sources I’m aware of as well.
If your 4" latex core is still in good condition and doesn’t have any significant impressions or soft spots then you could certainly add thicker memory foam topper (probably 3") to see how it worked for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) but instead of just adding a topper I would be tempted to add a transition layer of a more resilient material (such as latex) in a medium firmness range on top of the firmer latex core (probably in the range of about 2") and then sleep on this combination and then use your experience on this combination and the topper guidelines to choose a final memory foam top layer. You could also use the memory foam topper you have now on top of this to see how it worked for you with the transition layer as well and this may also be a useful reference point for choosing a memory foam topper (if the one you have isn’t suitable with the transition layer underneath it).
Once you have the combination of layers that is working well for you then you can add a zip cover (probably a thinner stretch knit cover that doesn’t have a significant effect on the feel or performance of the comfort layers) to "finish your mattress since the cover you have (assuming you have one) may not be thick enough to hold the additional layer.
Of course your own testing and sleeping experience will be the only reliable way to know whether any specific combination of layers or materials will be suitable for you in terms of PPP but if I was in your shoes this is the direction that I would be leaning towards.
Phoenix