4 Day is a lot of time... Start from the top Narkanda, then kufri, Shimla, solan. If you are going Chail don't forget the resort in water.. Its amazing, Also, Chail is another route and not in one line.
Thanks for sharing your thoughts and for vacation trip resorts are best. I have also booked a fantastic resort in Shimla i.e. Toshali Royal View. It was really an awesome experience for me.