That’s a tough one. Normally I would say “single” if you’re not flying with a companion, but 4E is such an awkward island in the stream sort of seat with people passing you on both sides....

I did an upper deck window seat on the KLM 747 once because I, too, wanted the upper deck experience. The climb over my neighbor was not a big deal (and I’m no gymnast), but this also was a westbound flight and sleep was minimal. Are you flying RT in J? If so, I would do 4E on the eastbound (hopefully less foot traffic if everyone is sleeping) and UD on the return...