Is this a terrible idea - afraid so. Your boars , even neutered, in the presence of females would go ballistic at each other.
You could have two herds of a neutered boar + females (you might be able to neatly divide them into two lots of B+S+S+S , or you might find the sows dynamics are better in a B+S+S and a B+S+S+S+S )
Are the sows related to the boars? Have you asked the vets about neutering? They'll give you a guide to the age and weight they want before your boys go for their little gentlemans operation.
I have a neutered boar with two sows and I'm going to try again to introduce our two new girls. I think the two alpha sows need some 1:1 time, couldn't do it this weekend unfortunately.
It's a lovely idea to have boars and sows, it's how they would live naturally. Keeping two boars together when they get to the Terrible Teens and there are sows around is do-able but you'd need to keep them completely away from the scent of the girls or they might turn on each other. Have you planned your living space for 2 groups? And have you got enough laps in your house
The boys & girls are kept v.v. Close to each other at the min - don't have the room not to be, the boys spend most of their time sniffing & looking at the girls but the boys are fine with each other.
I wonder if to get them neutered anyway & see what happens with them all living together - if it doesn't work then will split again. I'm worried we are going to have an accidental pregnancy if we're not careful - when we put them outside (well, when we get them in), we have had a few escapes & it's only a matter of time before a boy & girl escape together. This doesn't happen on my watch btw, it's Dh!
They might be fine at the moment but when the full on Terrible Teen Puberty hits these boys......................well, just be prepared!
You'll need to clue DH up on the pigs not having a Meet Up on His Watch, you just don't want to risk it. There's loads online like Guinea Lynx, Rodents With Attitude, Jackies Guineas about groups of pigs.
My sows (I have my original 2 + the neutered boar and my 2 new girls) are not getting along in a group. So , I have decided to keep them as two groups and ordered a 2nd run+housie. Cheaper than a Vet bill I told DH
My Gingerpig was about 6 months old when he was neutered. He could have been done a bit earlier but we were in no rush. I think it's down to weight really. Your vet will be able to advise on that. Mine said about 5 months old.