I have two working collies who are superb workers. I would never let the younger one be with sheep unsupervised.
Prior to being a sheep farmer, I had pet dogs, mainly collie crosses, who I taught it was naughty to even
look at sheep. They were superb, would come back to me as soon as they saw any sheep, and keep their heads behind my legs so they wouldn't look at the sheep. Bless.
But I would still never have let most of them be with sheep unsupervised.
So what I am saying is that you can train your dogs to behave around sheep when you are with them. But with 4, it is, IMO,
highly unlikely improbable that it will be safe to leave them loose with the sheep if you are not present.
You may be lucky and have one, or even perhaps two, that can, in time, be trusted with sheep when you are not there. But only on their own with the sheep, not with the other dogs and with sheep.
Get yourself a copy of
Barbara Sykes' book Understanding Border Collies, and read it. Twice.
I am sorry to be so blunt, but to think that
any four dogs will be safe unsupervised with sheep is misguided; to think you would achieve it with four collies unrealistic.
It is better that you get your head around that now, before you get the sheep, than later, after you have had to learn the hard way.
And don't forget, once your collies have learned how much fun it is to chase sheep, you can never ever let them off a lead safely in the countryside ever again.
I would consider other livestock, not sheep, in your circumstances.