Finding a mattress that will fit two people may be difficult given the dimensions of the cargo area. I believe the distance between wheel wells in a Gen4 is roughly 36 inches, so that would be your primary limiting factor. Inflatable double mattresses are typically 54 inches wide. Some lightweight camping mattresses may be slightly narrower, but the narrowest camping double that I could find is 40 inches (the Big Agnes Rapide SL Insulated sleeping pad measures 40” x 72”). I would be interested to hear thoughts from others on this topic — maybe someone else has found a mattress that works? Another option you might consider buying a large piece of memory foam or polyurethane foam and cutting it to fit the shape of the cargo area… Just an idea
IMHO, an inflatable mattress with a built-in pump, battery powered - of course, would be the way to go. It'll conform around the strut assemblies, unlike your hips or back. You would be able to adjust it's firmness, but you'll need to deal with temperature drops = deflation. Ergo the built-in pump, if you need it to firm-up in the middle of the night. Easy to stow away, so it doesn't look like a "bum-car", if people look in. Hope this helps....
