My wife and I (both in 60's) will fly into Kathmandu in February after a 3 week tour of Bhutan, before flying on to London.
Thinking of stopping in K for a few days - maybe just relax (in and around a nice hotel ??), maybe get the best sight of mountains, maybe an easy one-day trek to experience real Nepal.
Maybe even all three - which might justify more than 4 days - but nothing too tiring after a busy, buzzy tour.