Have you tried the original cushions with the 4" memory foam on top of that?
It sounds like the memory foam may not be good quality. A good memory foam will be around 4 lb density or even higher. Density is what gives support. Healthyfoundations.com is a good website I've bought foam from, and I've bought a good one from overstock.com (the density was listed for that topper, but some don't say and I avoided those). I would only use 2 to 3 inch thick foam though; 4" would be too sink-in cushy for me.
Another good possibility is a latex topper; look for Talalay latex (good quality). We sleep on a memory foam topper in both the bedroom and the trailer, but I use latex pillows... aaahhhh!