Is this sentence correct?
I was his teacher for [his] fours years at this school.
Do I get “his” in the above sentence?
‘his’ is incorrect.
Strictly speaking, you need one of these:
I was his teacher at this school for four years.
I was his teacher for the four years that he was at this school.
When you want to use words/phrases for time and place, the usual order is to put place first and time next/last.
Like I did.