Salutation for three or more professors

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.

What about a "Dear Sirs"? It is used a similar salutation here in Italy, but maybe is not suitable for your purpose. :-)

Either of the first two versions would show proper respect for the recipients; neither would violate any rules of the language or any conventions of letter writing. The second version, however, has the obvious disadvantage of being lengthy and cumbersome, which favours the first one. As has been pointed out in a comment by Mr. Wayne, the first version is analogous to the well established practice of using Messrs in salutations.