Hard to answer your question till we don't know flight schedule to Ktm but I plan like this roughly:
Day 01: arrival Kathmandu (sightseeing if you arrive early hours)
Day 02: Hike to Chisapani
Day 03: Hike to Nagarkot
Day 04: Drive to Bhaktapur and sightseeing, fly back home if your flight is at late afternoon.
it is better if you allow at least 5-7 days for short and perfect trip to Nepal about 6 years ago