If a person suffers from wiswas in number of Rakat. He doubts whether this is 4th or 3rd rakah but he has absolutely no gut feeling to tell which so should he assume 3 or just continue it as the 4th?

If you are suffering from waswas then disregard it, and if you don't know at all then continue the salah upon the lesser count of rakat. So, if you are doubting, did I pray 3 or 4 rakas, you are doubting the 4th, so 3 are certain therefore pray 1 rakah.