i have a futon couch which slides out to make a full size bed.  the whole thing weighs a ton.  don't know what is in the filling, but the futon is 4 inches thick, very stiff and unwieldy.  any suggestion to make this a more comfortable bed will be welcomed.  i am partial to latex.

If you like latex, then try a latex topper. But if I am correct, that the reason you aren't liking it is because it is not providing support for your back, then the latex topper won't help. You will still sink in where you shouldn't sink in. Actually the latex topper might help a little, but probably not enough. Worth a try. You can always use the topper on the extra firm mattress you will need to buy sooner or later. (The firmer the better, as that means less cheap poly foam inside.)