It's even weirder than just 1, 2, 3, 4. It starts at 000 and goes to 4, with stops along the way for .5, 1.5, 2.5 and 3.5. But since you can't rely on the number on the tag even with conventional sizing, it doesn't bother me. It's easy enough to check out their size chart online and find your "Chico's number."
Remembering the recent thread(s) about the lack of options in larger sizes, I'm surprised nobody mentioned Chico's. They go up to a conventional-size 20.