If these were my birds, I would remove the attackee ASAP

There are arteries in the toes that, if seriously bitten, can cause a lovie to bleed to death in less than a minute. You are correct in using the term "abuse". Would you leave a human child in a bedroom where three other children regularly ganged up on it? And do you think that a bird that is being attacked is suddenly going to become part of a bonded pair with one of the attackers? There are several possible explinations to this situation, including that you don't have 2 cocks and 2 hens. Hens get very aggressive when they start thinking about starting a family, and will try to eliminate anybirdie other than the "object of their affections"

They may be small, but lovies are perfectly capable of killing a bird much larger than themselves, let alone another lovie. IMO, if you don't remove the birdie, you will soon have 4, not 5, lovies

And don't assume that you can just move the attacked bird to the masked birdie's cage. Lovies decide who they will share their space with, not you. And there is a possibilty that once you do remove the survivor (if it is still alive), two of the remaining birds will gang up on the third. Three is a dangerous number in a lovie cage

Good luck with your flock