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.