Females picking on a newbie

[Jojomassey]

I am very new to raising chickens but loving it so far! I welcome all the chicken knowledge I can get.
I have 2 6-week old females (bought together) and 2 3-week olds (bought together) we keep them mostly separated but allow them supervised time to slowly get acclimated with each other. The two oldest girls ONLY pick on one of the newbies. They don’t seem to notice the other. Is there any rhyme or reason to this? They are different sizes so we have no intention of adding the littles to the coop/run until they get bigger.

 

[aart]

we keep them mostly separated

...but in sight of each other?

Here's some tips about....
Integration Basics:
It's all about territory and resources(space/food/water).
Existing birds will almost always attack new ones to defend their resources.
Understanding chicken behaviors is essential to integrating new birds into your flock.

Confine new birds within sight but physically segregated from older/existing birds for several weeks, so they can see and get used to each other but not physically interact.

In adjacent runs, spread scratch grains along the dividing mesh, best if mesh is just big enough for birds to stick their head thru, so they get used to eating together.

The more space, the better. Birds will peck to establish dominance, the pecked bird needs space to get away. As long as there's no copious blood drawn and/or new bird is not trapped/pinned down and beaten unmercilessly, let them work it out. Every time you interfere or remove new birds, they'll have to start the pecking order thing all over again.

Multiple feed/water stations. Dominance issues are most often carried out over sustenance, more stations lessens the frequency of that issue.

Places for the new birds to hide 'out of line of sight'(but not a dead end trap) and/or up and away from any bully birds. Roosts, pallets or boards leaned up against walls or up on concrete blocks, old chairs tables, branches, logs, stumps out in the run can really help. Lots of diversion and places to 'hide' instead of bare wide open run.
Good ideas for hiding places:
https://www.backyardchickens.com/threads/a-cluttered-run.1323792/