If you've truly given him specific things to improve on, then its time for the performance review/"the hardest thing we have to do" speech.
I know that it's hard to tell someone they are no longer welcome in a project - especially if they feel it is their project - it's hard enough to tell a band that you're firing yourself. But it sounds like the rest of you have come to the point that you realize "this" isn't working. Three of you feel the solution is to move on with out the other guy. Why would you continue as it is?
The three of you need to pick a BL - even if your process is primarily democratic, someone needs to make final decisions on things and manage personnel decisions (my band is like that). Then decide if you want to do it together or have the elected BL do the dirty work. In either case, be respectful and honest. If the guy on the oust gets weird about, you all just quit "his band" and miraculously find each other next week and start a "new" project.
To me, if I'm not wanted in a group situation, then I don't want to be there. If my band fired me without any previous discussion, I'd be shocked, embarrassed and disappointed, but certainly wouldn't be dramatic about it. If I had been given opportunities to fix my deficiencies, then either A, I was too lazy to do so or B, my best isn't good enough and I'd probably understand. So while I would expect there to be initial shock and minor drama, if the person is anywhere near emotional adulthood, it should be "nice knowing you, maybe we'll see you again sometime" after the initial shock.