I actually introduced all four main characters, although two of them briefly, in the first chapter of my current comic. In the second chapter, the briefly introduced character got the limelight and got properly introduced. With each chapter, I usually present a few characters that are secondary to the story, but to help understand the main character's back stories and personalities and such, and I guess that will happen for a few more chapters =)
I've introduced main characters to stories pretty late in older comics, but I think I prefer to introduce them early and flesh them out as the story goes - I've made this character, let me tell you (almost) everything about them XD