Short thank-you comments are generally frowned upon by the Stack Exchange. The 15-character minimum aims to keep comments substantive.
Some other things to consider:
- On the main site, only five comments are shown on the page. Adding a "thank-you" comment could push down a more meaningful comment and make it harder to find on the page, like the comment that is hidden here:
Moderators get flagged every time a question gets 20 comments or more. So, by using four comments to thank four users, you are using 20% of that quota just to say thank you.
I realize this isn't addressing the exact situation as you describe, but I still think the overall philosophy applies:
If you really feel compelled to leave a thank-you comment, I suppose you could so do "responsibly" by saying thank you – but then go back a day or two later and delete those comments. That way, the four professors get notified of your gratitude, but the thread gets tidied up without any moderator intervention, too.
Note also, though, that a busy SE user might prefer to NOT get such a notification in their inbox. It can be a distraction to go check a notification that says nothing more than,
Many thanks, J.R. :), with the name and emoticon added just to reach the 15-character minimum. I know I have other things I'd rather be doing.