is it possible to install a custom moonroof in an 04 mazda 3 4-door? I've always wanted one in mine

A dealership wanted my business so bad that they said they would put one in. Sounds like it is possible and I have heard talk about it on different forums but if it is not the factories moonroof... chances are it could leak which is never good.

So you mean leak like air comes in even with the glass closed?

I believe that's what he means. But the worst are the potential water leaks.
I'm in the same boat as you are. I wanted a roof so bad in my next car, which turned out to be my '04 3, but no roofed cars wereavailable as I wanted @ the time. I bought mine near the end of the model year and they weren't making '04's anymore, nor had the '05's come out eitherby then,
AM pop-up roofs used to be notorious for leaks. If that is what they are offering you, I would stay away based on experiences friends of mine have had w/them in the past.

yes its possible.but the problem your going to face in the future will surely give you a headache. water will surely leak if the moonroof is not proprly installed. Since you made some cuts on the body, rust can also start from it.