Hi Sheetal.. Flights to Udaipur are a little costly.. So, you might want to try the train option for one side. But if time is a constraint, you can book flights for both ways. I feel covering Udaipur to Jaipur is 4 days is difficult. You can split it up into to parts - either cover one of the below two paths:
1) Udaipur-Kumbhalgarh-Ranakpur-Jodhpur/Chittorgarh or 2) Jaipur-Ajmer-Pushkar-Ranathambore.
Rajaethany has a lot to offer. So, I would say is give yourself enough time to enjoy it.
From Mumbai Direct flights to Udaipur.
Must visit Udaipur City Palace ( Almost will.take 4 hr from inside ) then Karni mata ka temple ( Here you need to go by ropeway) , Saheliyon Ki bari ( Evening garden) and then in night go to Lake pichola with some grrat restro near about it. Next day you can go Jagdish temple, Fateh Sagar Lake, Vintage Old Car Museum.