Hi SleeplessinNJ,
Unfortunately there isn’t really an effective way to firm up a mattress that is too soft without removing and replacing comfort layers that are too thick or soft or a support core that is too soft because a firmer topper will tend to “bend into” any softer layers or any soft spots or dips underneath it and the results are usually partial or temporary at best and at worst they can make your support issues even worse.
There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
It’s quite likely that your mattress has developed soft spots or “virtual” impressions after 4 years of use because of the lower quality/density materials that are likely in the pillowtop but there are some suggestions in post #2 here that may be helpful in situations such as yours.
Phoenix