So that's a tough situation. How much of the creation of these characters was she involved in? I would say make sure all of your efforts to contact her have been well documented so that if she reappears you can prove you were not doing anything sly.
Do one better, state that your comic is based on an old rp you used to be a part of that disolved years ago on your "about" page. I can't speak for any other comic maker out there, but I would see no problem with what you were doing unless you tried to deny her part in things.
Selling it seems like a thorny issue. I'm not sure how the law would apply to rps and whether or not being involved together makes you both partial owners, but I would honestly consider not trying to sell this unless you've somehow got her blessing. Because if she pops up some years later and you're selling stuff that includes characters she never gave you permission to use in the first place, I don't see how you justify not giving her a cut. Granted, her cut should not be anything approaching half, but if you are using her characters, she's entitled to something and you'd be a d**k ( if not legally wrong) to think otherwise.
You say that "this is because of reasons" but i'd seriously consider altering some of her characters to try to avoid this potentially messy issue. To be frank, you currently do not have any actual work up and so it's not like it hurts anything (you can't hurt what currently doesn't exist) to throw some changes in there. Mind you, simply doing recolors will look pretty sleazy so I recommend you do more than that- which shouldn't be hard if you are basing it off a four year old picture. You'll probably be retooling a bunch of s**t from your rp to better fit the story you want to tell anyway, so dont discount this option completely.
I wouldn't do it. I've been on the other end of this issue (though there were 3 people involved) and I and the other person in the same situation were not happy about our characters and essentially a good chunk of our writing being used and claimed as someone else's for a book. We approached the person doing so and she assured us that she would remove our characters, but only really changed the names. How could the story be the same otherwise?
I don't think the theoretical book will ever be published, though. The girl in question was the weakest writer of the three of us, and I'd hardly call us skilled. XD
If you plan to stick to the RP plot, you're using more than just a few characters. Even if you're not, you're using someone else's characters without their permission... It's not a good thing to do.
I suggest taking some of the aspects you like about these characters and using them to build your own characters instead.