We have a 4in memory foam topper on top of the original TM mattress that we bought from BB&B (for tooo much). I wouldn't do it again, although my DW loves it.
One problem for me is that the mattress retains my body heat in all that foam that surrounds me. So far, with the cooler weather, this has not been that much of a problem, but I think that during the Summer months I might have to find a better solution.
A second problem is that as we crawl into and out of the TM fold-out that is our bed, our knees sink into the mattress just about that 4in each time which makes for a tangle of sheets during the night (we have to get up occasionally
) However, I have found some tarp clamps (Coleman) which along with some some elastic tapes might help to retain the sheets.
I don't mean to say that any of the above outweighs a good night's sleep in my estimation.