Prepositions are trick but if you are talking about being in/at the fourth year at work/school or something like that you should use the preposition a.
Nell'anno is used to say things like 'Rome in year 1838' = Roma nell'anno 1830 or 'In the year of the comet' = Nell'anno della cometa
IN is correct. IN the fourth year.
Explanation: The fourth year is a mass of people and individuals go IN this mass of people.
Examples: I am IN the army. I am in the police. I am in the chess club.
If you are talking about levels, on a computer game for example, then is is AT. I'm AT level 7 of Call of Duty etc etc
I am reading a book about an American family which is spending a year in Italy. The children are in school and they refer to grades as years. First year, third year, etc. That is the only reason I didn't get it wrong because of that book. It is called Il Bel Centro. I am really enjoying it, and learning some of the nuances of Italian I haven't learned yet from DL.