You may not think it was wrong of him to "educate" 4th graders about the truth of Santa, but some parents would think it was very wrong. He is working with a large number of students from just as many families so he needs to be aware of that. He should think before he speaks. It is not the music teacher's place to "educate" about this particular subject. He was talking about leperchauns and should have left the discussion at that. Anyone working with this age kid should be aware that they are all at different places developmentally and some may still be hanging on to what some believe to be childish things (they are, afterall, children). He should respect that. Its not like the belief or nonbelief of Santa is a required skill in 4th grade music.
I don't prescribe to the belief that a teacher's knowledge and education should only reflect the subject they teach. If the class is studying/singing a song about mythilogical creatures and his students question him, I would hope he was willing to educate them with a general lesson. How sad it would be if a teacher was not allowed to discuss anything other than the subject they teach. We just disagree, I respect that some teachers have a vast knowledge of may subjects and are willing to share that with their students. I hope they would keep it age appropriate and in this case I believe he did, and you don't. I have a 9 year old 4th grader (who does still believe, I think) and I wouldn't have an issue if he was sitting in that class.
I will take you along to meet or watch him and you'll get it in just a few minutes.
Regarding the first bolded part - I also have two younger kids in the school so my concerns are personal.
As far as my relationship with the principal goes, I have known her since we were college age. We eventually went our own ways and it was quite amusing that we've caught up again. Currently I'm the P&R director in our town and the school is considered the community building (after hours of course). But since school functions take priority and afterschool programs are part of our offerings, I meet with her once a week to discuss room availability, transporting the kids, procedures, etc. While I'm there, we also talk about the school since I'm a parent of three there. She has done a great job turning the school around and is very adamant about a home-parent-school connection. She often asks if I've heard of any concerns or wants amongst parents she usually doesn't see or wishes would be more involved. She asks because she wants to address it and keep the place moving forward. There are many people besides myself that she speaks to about this. Basically, she's a communicator. I've never complained about a teacher to her, which should tell you a lot regarding my feelings toward the music teacher.
Regarding the second bolded part, we will have to agree to disagree on this one. I've said it many times that I don't believe it is his job to educate my kids on certain things - this is one of them. Do I think he did it because he's mean? No. I think he did it because he wasn't thinking...which is a recurring theme with this man. I guarantee if I mentioned it to any of my kid's classroom teachers, they would roll their eyes and not be surprised.
Either way, I'm probably going to do nothing. But my patience and confidence with this guy has worn very thin.
I think if you have an issue with the fact that he let the cat out of the bag, by all means talk to him about it. However, since I don't believe he did anything inappropriate, IMO it doesn't warrant a complaint to the principal. I also don't believe any parent should be complaining about what a teacher may, or could say in another class IF it has not happened in that class. As it stands now, he hasn't said that to your younger kids, why would you complain that he might. Could you imagine what it would be like if every parent called the principal with complaints about things that haven't happened, and may never happen.
I don't know you but from your posts it just sounds like you have no respect for this teacher and you are looking for something about him to complain about and if I was the principal friend (and heard your previous comments about him) I probably wouldn't take this complaint very serious if you came to me in a proferssional capacity.