Hi and welcome
That looks like vent pecking, which is damage done by another chicken. If you look it up you should be able to find a lot of info. It is usually associated with commercial laying birds and is due to over crowding, stress or other factors in their living conditions.
If you have 1 bird that is OK and 3 that are being attacked, you may have a serial pecker and she will need dealing with. Worst case scenario is that you will have to cull her as it is not fair on the rest, but you could try anti-pecking rings, which fit onto the beak.