You've obviously had some previous experience. I'll offer some thoughts.I don't think I've ever had a chicken do this?
1. You have seen this before but you did not have the larger eggs to compare them to so it wasn't so obvious. They get larger so gradually you don't necessarily notice. It's like the egg shell color often gradually gets lighter the longer they lay but it's so gradual you don't notice. You can try Aart's idea of weighing them to see if they are getting bigger. I like keeping a small one in the fridge for a month and then compare the size.
2. The younger the pullet starts to lay the smaller the egg will be. Did these four start laying younger than your previous pullets?
3. What breeds were your other pullets? Some breeds or hybrids like ISA Browns start out laying larger eggs.
4. Are you feeding these differently that the previous pullets? The higher the protein content of the feed the larger the eggs should be.
5. Do you still have some older ones laying larger eggs. Anything changed with those eggs? If not, probably not a disease.
It is possible for one pullet to have an individual issue that could affect egg size but it would be a really strange coincidence if it affected all four. Not going to happen. I suspect there is a very logical and non-threatening explanation.