this isn’t a notation related question, but i don’t know a forum elsewhere that can answer this question i think 
what are the highest notes possible on each string of a violin?
The highest notes are harmonics but you probably mean notes played with finger down.
As far as I can remember Prokofiev notated a G two octaves and a minor third above the open E string in the ballet Romeo and Juliet. Theoretically you can go further to the end of the fretboard but this becomes more difficult and the sound becomes thinner. In the Prokofiev example the extreme high notes are used as a “silver shine” one octave above the actual melody line.
From my experience for melody lines I would notate a maximum of about one and a half octaves above the open E and A string and one octave above the D and G string (high ranges on the G string can be used for a rich espressivo sound).
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thanks @vadian !