M.L.
Do you dress the twins alike? If so, maybe dress your oldest in matching clothes sometimes as well, or don't match the twins up as much.
Do you treat the twins like they are a pair? I know a mom that does this almost treats them like they are one person and I can see how it drives her other children crazy.
I think it's common for siblings of twins to feel jealous and hurt because they can never be 'as close' to the twins because they are such a pair.
I would encourage the girls to all play together apart from each other. Take one twin and the older sister out for breakfast, another day, to this as well and switch which twin you take. Introduce new hobbies they all may like. Help her foster relationships with both girls, but sometimes you may have to separate them a bit to do that otherwise she may eel like she is always trying to be part of the clique.
This advice is gearing more towards dealing with infant twins and sibling rivalry, but I think some of it may still be of help:
http://www.twinslist.org/sibling.htm
http://www.suite101.com/content/sibling-rivalry-among-twi...
Also, explain to your daughter they they are 4 years old, they are on a different level than she is anyways. Help provide ways for her to cope with her anger and help her realize that they won't want to be friends with her if she continues to be mean.
Expert advice for that here:
helping the angry child
http://www.askdrsears.com/html/6/T061700.asp
sibling rivalry:
http://www.askdrsears.com/html/6/T064200.asp