Hi joebk,
IMO … not only is this “thick enough” but in combination with your pre-existing comfort layers you have 6" of soft foam in the comfort layers which is likely to be “too thick”. This would be especially true when the lower density foam softens under the greater compression of the heavier areas of your body and there would be significant risk of alignment issues with this combination. Of course each person is different and your actual experience may not reflect this but it certainly seems to me … for the reasons that I mentioned in the earlier posts and in post #8 here … that you may be too far away from the firmer support layers in your sleeping system to provide good primary support (the support that “stops” the heavier areas from sinking down too far).
With a 4 lb softer memory foam topper … I think that the bigger issue would be that it may be too thick rather than just too soft. I would hesitate to use a topper this thick in almost any circumstances when a mattress already had 2" of soft foam in the comfort layers. It may be “OK” in the shorter term but the risk is much higher that it won’t be in the longer term or even over the course of the whole night (memory foam can “creep” or keep on sinking over the course of the night) after the initial and ongoing softening that is more likely with 4 lb memory foam.
I should also mention that your experience with sitting on the mattress will have little to do with its suitability for sleeping because sitting concentrates more weight in a much smaller area and this is not what you will experience when you are actually sleeping on the mattress.
The bottom line is that if it works for you then there is no arguing with success or personal experience but it would certainly be a much more risky choice both in the short and long term for the majority of people IMO.
Phoenix