Since you can't go pro until after your 3rd year, your 4th year would be in the pros and then, assuming a flop in the pros in the 1st year, a player could go back to college for one year.
Why would anyone do it?
And typically the guys who go pro after 3 years are the best of the bunch so, from a practical standpoint, how many guys would there be who would do it?
And the super-senior type COVID exceptions ending after the 2025 season, the same ability to only play only one year still applies.