I work short early shifts in the holidays, so I start work before they're awake - leave at 5:30am, work 4 hours, get home at 10:30am. That works fine as the kids get up about 7am, have breakfast and play on screens, read or draw (depending on which child). ideally they'd lie in, but mine appear not to be programmed that way... I work 9 hour shifts the weekend at one end of most holidays and most bank holidays, so it balances with hours. Have a DH who works office hours so we cover holidays without needing time off specifically for child care (though we take time off to do stuff and go on holiday etc etc. We have no child care other than both parents.
Is your children's dad totally absent? Doesn't he have them at all?
Presumably you and your sister could help each other somewhat, at least being an emergency contact, as it sounds as though you live close together.
Also you may not be a single parent in 5 years time, as well as not knowing how your children will pan out...
Do you envisage leaving the eldest home alone while the others are in child care? If you're paying a babysitter for the younger ones you won't end up leaving a 12 year old home alone at all - eldest will have babysitter due to younger ones, younger ones will have older siblings...
I don't think you've thought it through if you have 4 kids under 7....
Presumably at the moment you don't work?