Salutation for three or more professors
Solution 1:
I would use the salutation
Dear Prof. Smith, Prof. Johnson, Prof. Lee, and Prof. Chun,
(note the “and”).
Other salutations might work, but this version gives adequate respect to each addressee, without being inordinately lengthy.
Also, is the email directly relevant to all four professors? Perhaps you should consider “CC’ing” someone that’s not so relevant (in which case the CC’d person need not be addressed in the salutation)?
It’s my first answer here so any feedback is appreciated.