I graduated from college in early May. I was a full time student for four months Was I a full time student for 2021?

Yes, if you class load was considered full-time then you were a full-time student in 2021 because you were a student for 5 calendar months.

"A full-time student is a student who is enrolled for the number of hours or courses that the school considers to be full-time attendance.

To qualify as a student, the person must be, during some part of each of any five calendar months of the year:

1. A full-time student at a school that has a regular teaching staff, course of study, and a regularly enrolled student body at the school, or
2. A student taking a full-time, on-farm training course given by a school described in (1), or by a state, county, or local government agency.

The five calendar months do not have to be consecutive." (IRS)