I put the green side down. Then I put my mattress pad over it and my older mattress. I don't think the topper will slide but this prevents that.

It will roll, but is VERY bulky. You would need to find or make a storage bag for it. It comes vacuum packed in a bag, and won't fit back in once air gets into it. You might prefer getting blow up mattresses for sleepovers, they take hardly any room when not in use.

You could cut the foam easily with a pair of heavy scissors or so, but you cannot resize the cover, so cutting it is a bad idea.

I have had mine for 2 weeks and it fully expanded to 4" by the second day. I slept on it the first day and it was great.

Mine just took a little while to expand after I took it from the package. Mine did eventually expand to its full size so I hope yours does!

There isn't much difference, but if you read the specifications, it will tell you what exactly the matress topper is filled with. An example is "1.5" memory foam + 2.5" high-density foam". If you're looking for a more softer matress with more memory foam, look for a product with less high density foam, and more memory foam. If you're a fan of a firm matress topper, one like this with more high density foam than memory foam will be best for you. The memory foam is what forms to your body, creating a softer bed. The high density foam is just a regular foam, like any other normal matress topper. Hope this helps.

yes true to size of mattress

I would think so. The four inch version would hold its own and not shift over the futon mattress. And, together the two of them would fill a fitted sheet pretty well.

Keep the topper covered with the mattress pad and sheet. And follow the manufacturers recommendations.

Yes. Fabric.