Before I get into the knitty gritty of the problem, I think it's best if I give some background first as I'm a newbie here.
I live in the UK, I'm 16 years old, and I keep four chickens in the back garden. They have plenty of space to roam and forage and we give them treats daily (scraps from the kitchen, greens from the garden), and keep water, food and grit in constant supply. The run itself is about 8ftx15ft and the outer pen (which we open to the chickens as early as possible) is twice as large.
The breeds we have (since Pullets) are a Partridge Amber (four years old, the boss, called Marigold), an Amber Lee (second in command, a year old, called Amber), a Speckledy (year old, called Speckles) and a Bluebelle (year old, called Bluebelle). The last two seem to be joint-bottom of the pecking order. They have all been together for about a year.
Marigold has always been the boss. Before we bought the new three chickens (Amber, Speckles, Bluebelle), all the other breeds we had were the Bovan Goldline. We had similar problems with them, but strangly it was Marigold getting pecked (she has totally recovered). Now, however, all three younger Hens have been pecked quite badly by each other (thankfully not to 'bleeding point'). By process of deduction I know for a fact Bluebelle is not pecking any of the others and she has the worst... 'damage'. It seems she is being picked on by all the other Hens. Speckles has similar feather damage, and she only pecks Bluebelle as she is scared of Amber and Marigold. Amber pecks them both, and Marigold all three but not a great amount. This means that only Marigold would be pecking Amber, who still has significant feather loss.
The pecking itself is not constant and in excess, and usually the victim moves away quickly when they realise what is happening. The balding sign of feather pecking is mainly on the legs, vent area and crop of Speckles and Bluebelle, and only on the neck and crop of Amber. We have tried seperating the main bully (Amber) which didn't work, and then tried plastic nose clips/rings on Amber and Speckles (I didn't want to put one on Marigold as she is quite old) which allowed for some feather regrowth. However I noticed them still trying to peck. We have taken the rings off now because they gradually wear away, and have not replaced them as there is litle difference in feather quality. We have also sprayed them a few times with an antibiotic/anti-cannabalism liquid designed for poultry which gives the feathers and skin a brown appearance. Similarly this didn't have much effect.
So, I am unsure what to do. I don't know whether to allow permanent access into the outer pen (which is secure from foxes) or to seperate all four of them until they have regrown all their feathers. The routes we have taken so far seem to be having little effect, and I would really appreciate some help from more knowledgable chicken keepers on this site. Thankyou!