So going over chapter four... We really should get to that, maybe after it's all caught up we will, she says she'll be 28 in February which Mark says will make her older than him. I believe he does, indeed, say he's four years younger (or will be) at that stage. My Japanese is only so-so, like I'm not on a native level by any stretch of the imagination, but it feels like Author-sensei might have been inconsistent. In the chapters we missed, it definitely says she is 'no older than 26' in the pages we missed in chapter 5... Although we're going to adjust that slightly for consistency, because it doesn't quite match up else-wise.
Edit: pages we missed in ch 5**