Covering East and North Sikkim in 4 days is quite difficult considering the terrain. You can cover major attractions and enjoy your time in Sikkim. You can download Gangtok PDF guide and North Sikkim (Read More) PDF guide from the following links respectively to help you in your journey - https://www.holidify.com/places/gangtok/
https://www.holidify.com/places/north-sikkim/
1 night in Lachung doesnt worth. I stayed for 2nights so i got ample amount of time to visit the village and get easy with the locals. They are wonderful.