I am strict on screens but the OP has had some nasty comments. No one wants to listen to people that speak to them like that.
If you're interested, OP, my dd is 9 in y4 and has only occasional access to an iPad. She does play some games on long car journeys as she doesn't travel well in the car. Anything like Roblox would be a no for all the reasons others have mentioned. I do appreciate that the child's peer group is relevant, if they were all on it, that would be difficult. But that won't be an issue for you at 4.
I won't be getting dd her own device until the absolute latest I can get away with it. It's ok at the moment but imagine things will ramp up next year. When I do, it will be limited and very closely monitored. Even then I still know there will be problems. Look at all the problems adults get into on WhatsApp with group chats. Imagine kids trying to manage this.
If it were me, I'd put away the iPad for a few years. If I kept it, I would delete Roblox, explain it's not safe for her age, and accept she will be upset for a few days. Hard, but necessary. Even Minecraft is way too much for a 4yo imo, even just visually. Keep things simple like c beebies apps. But if you insist, it's better than Roblox. I would add s
I would spend some time learning about on line safety and parental controls. I think someone provided a link for this earlier.
Allowing a 12 year old to make decisions on your daughter's device or time online with her unsupervised is unlikely to end well. A 12 year old is not yet sensible or safe online. So I would speak to her father about that. It's not a criticism of her sister, this would include any 12 year old.
The latest technology for our children has not been a good step forward. I appreciate it's tempting, as doing without is more work for us. I'm sure it's very tempting to her father to let her and 12 year go on screens together or separately. It's a tricky age gap to manage. But it's not good for her.
I will avoid it for as long as possible. smartphone free childhood on Instagram which gives lots of information and community for anyone concerned about the effects of children being on line and on screens.