Hi Sneha, Yes spiti can be done in 4 days at an very low budget. I Spiti in winter last week of december 2018 at an very low cost.
Mode of travel should be - HRTC government buses which are reliable and safe
Stay at homestays who provide dinner,breakfast and accommodation inclusive. Message me for more details. Wish you the best,cheers !!!
The food in spiti is too costlier than other places in himachal as spiti is isolated from our world. So for budget cutting you shall carry sone food by your own that will survive for 2 3 days. And fellow travellers suggest HRTC buses are avavilble from manali morning 5 AM which costs only 300 INR. This bus is almost full for every day so i suggest get your reservation done online from HRTC site prior to leaving.