Well, my DD is not quite 4, but I'll say that she works in leaps and plateaus (or, rather, what appear to be plateaus). By her 3rd birthday, she was reading at a K/1 level. Two months later, she was at 2nd. After another two months, she was at 3rd. And two months after that, she was easily reading some 4th grade level books. Since then, there's been no obvious progress in reading--but why a three year old would be interested in more advanced books than that is beyond me. I figure (and I hope this isn't just my head in the sand) it'll probably be a couple of years before she wants to read a 5th grade level book. It's not just a matter of ability, but also of interest. Likewise, with the math--she suprised me out of nowhere counting backwards and skip counting before she was 3, and then was totally uninterested in numbers until recently, when she began doing basic arithmetic in her head.
I think this is how gifted kids tend to do it: they become interested, they learn like crazy, and then they move on. You have to expect a lot of jumping around.
Also, I'd be sort of cautious of categorizing a child as being at a K level just because she's able to do a K level workbook. I think Kindergarten is so much about learning to read and write and follow instructions that I'd be suprised to find much of substance in a K workbook. (Or maybe that's just more of my sticking my head in the sand.)
