The first was laid 2 weeks ago, the second 12 days ago and the new 2 have only been there a few days. I've candled them and this time written on in pencil the order they were laid. The first 2 are fertile and look like they're developing, the 3rd is starting to develop and the 4th is clear which means it is either newly laid and not started to develop, or it is infertile.
It looks like they might end up with 3 chicks! The 1st chick is helping them to hatch them and they are fighting yet, so I'll leave it in the same cage for a bit.