Newborn Fontanelle

My baby is 4 weeks old today and eats like a trooper, he's easily taking over 1L of pumped breast milk a day and is putting on weight nicely. Plenty of wet and dirty nappies too! However we've noticed that once a day, usually early evening his fontanelle seems really sunken... sometimes it only lasts literally 5 mins then looks totally normal again. Everything I've been told is that it appears sunken when they are dehydrated, but I'm struggling to believe that with my baby as he takes so much milk every 2-3 hrs. Then the rest of the day it looks totally normal. Has anyone else noticed the same?