@mummy917
I would happily take them if it wasn’t my weekend to have them as I know it’s not easy taking all 4, but it wasn’t even discussed as an option, he just chose not to take her at all.
Remember the saying "Short term pain for long term gain"? I know it's hard, but you need to be thinking in terms of the next 10 years, not the next party date. What you do (or don't do) now will benefit the DC in the long run. For the DC, even as hard as it is, they may as well learn now how unreliable their father is and missing a few parties is a simple way to learn that. If they don't absorb it now, the time will come when he will seriously let them down in a big way and they won't be prepared for it.
And for you, if you continue to bail him out all that will happen is that you will show him that he doesn't have to be a reliable parent, because you will pick up the slack (as you've always done). At some point, maybe not right now, you are going to want to 'make a life' for yourself on the days you don't have the DC. Perhaps a few days away, days in the city with friends, and even to have a 'love life'. And if he knows all he has to do is snap his fingers (ie not do what he should do) and you'll jump you'll find yourself jumping 6 ways from Sunday and you'll never get that independent life you deserve. And frankly, that you need.
Deep breath, squared shoulders. This is just another bump in the road.