Babies who don’t yet meet the 250g at 3 weeks of age can stay on with sows for a bit longer but they still need to be separated if they reach 250g by 5 weeks of age. Nothing you can about it though only hope the sisters and mum haven’t become pregnant by them.
No I don’t think it’s worth the introduction. Even if the two babies are fine by it, it’s a huge risk to the other boys‘ relationship, particularly as they are teenagers themselves.
If theyve always been inside then its a bit too late in the year for them to be able to acclimatise now, particularly with them being so young. It would probably be best for them to stay indoors until next Spring. Move them out when it gets warmer in April time and then they can stay out. It’s likely they will have to live indoors until next spring.
Cyril and Basil can live side by side with Jupiter and Apollo. Boars living as neighbours is usually absolutely fine.
But, its of course best for the girls to be at the bottom of a stacked cage, not the boys.
Boys who have always been around girls tend to have a less strong reaction, but if you’ve got to keep the two youngsters indoors, then they will spend months away from sow scents, so putting them into the shed next spring might cause issues - particularly as they will be well into their teens by then. Something you’d have to play by ear really. All I can say is that introducing sow smells to bonded boars who aren’t used to sow smells is risky for their bond