I'd like to at least think he's the same Pavel Chekov of the TOS universe, so I guess if I had to come up with an in-universe explanation for the four year difference in age it would be a transporter accident.
Wouldn't it be much simpler to assume Chekov simply misstated his age in "Who Mourns for Adonais," that for some reason he said 22 when he was 26? Maybe he wasn't using Earth years. Maybe he had some reason to pretend to be younger than he was.
The simplest solution of all would be just to ignore the "Twenty-two, sir" line in "Adonais" and assume he said 26 instead, or maybe 25 depending on where in the year it fell. Heck, there are plenty of other numbers and minor details in Trek that we simply have to ignore, like "James R. Kirk" and the timing discrepancies I mentioned in several above posts.
It would definitely be much better to ignore the line in "Adonais". I'd prefer it. He was just seventeen. You know what I mean. Or to give equal time, people can ignore the line about being just 17 in ST09. It's their preference.
I was only being facetious in making up my reason for the difference in ages. I mean, if one wants one, any reason works, really. It's sci-fi.