@MessAllOver
The point is that the OP's ex is 50% responsible for financing his children. So when he and his wife were discussing how to manage when the babies arrived and it was suggested that he should be a SAHP, he should have said, "No, that doesn't work for me because I won't be able to contribute towards my older children if I'm not earning and, as a responsible father, that's something I have to do". Then they should have come up with another solution.
And that other solution is?
For him to work and not her and eveyrone have less overall, so actually the older SC get virtually nothing anyway? Or so that his youngest DC’s bills can’t be met at their house?
Should both ex and SM said, we’ll both quit work and claim aswell and pay virtually nothing?
And if we are back to suggesting he should work around his wife’s hours, that largely depends on what she does, if she’s a nurse or something almost impossible for him to do.
If she works a normal 9-5 he could work evenings or weekends, but the weekends would probably mean he can’t see his eldest DC at all for the 25% of time he has them. I wouldn’t be expecting SM to have all 4 children alone over the weekend after working all week when 2 aren’t hers. Plus that’s not quality contact time for the 2 eldest with their dad. That household would be absolutely knackered doing that, I don’t think that’s in any of the kids best interests either, but people only seem bothered about the money here?
And if he could work evenings, then yes he should; but him finding a job to suit evenings he can do is no more easy than OP finding a job in childcare hours or the 25% of the time NRP has the kids, so she should also do this.