よんねんせい！ I don't think there are any counters in which 「よん」gets shortened to 「よ」。You either say it as 「よん」or「し」, at least when talking about the counter with the number four. EDIT: Pardon, I made a mistake. I just checked it myself and its よねんせい。Also, another counter in which よん gets shortened to よ is the one for hours, よじ。There may be more though. I'm sorry if this was confusing for you.

It is Ambiguous scene. Time 4:00 is "よじ" 4 hour is "よじかん". 4th grade is"よねんせい". but a class of 4 is"よんくみ". In japanese "4年4組" "よねん よんくみ" The 4th floor is "よんかい", too.