Hey B.,
its Just a stage,
ANd basically you need to BREAK the pattern.
Stop yelling even when your angry,
Make special time for each child.
Tell your 4 year old Leah is a GOOD GIRL.Just a baby.
Your 4 year old feels left out because your not including her,
and doing Joint activities with the baby.
things they can do together,
Play do and cookie cutters
Dip fruits into choclate or cream
read storys
ask the big one to read to the baby
I just think you need to be more organized and plan your days better.
the yelling has to stop , because your 4 year old hears it and it makes her repeat it and its making her feel insecure.
it also Obviously is not helping the situation.
your 22 month old isn't stopping, she is actually jusst building up a tolerance to the yelling, and it now means nothing to her.
a better way to handle the situation is to just remove her physically, and put her in a play pen, and ignore her for 5 minutes or until she stops screaming.
I usually say GO to bed, and my sons will run to their beds and cry it out ,
afterwards i go in and ask them NOT to do it again.
they agree, and we start all over again.
( of course they sometime repeat the act, but eventually they get it)
http://www.savvysource.com/activities/home
here is a website, if you need more I can certainly help you out,
Basically if you combine their activities they learn to enjoy eachothers company, and you teach them to share, and the little one will copy her older sister. and learn her good behaviors.
Finally I want to focus on the part where you say she is dangerous.
that concerns me,
now either your exaggerating, and she really is just not being watched properly
OR she has something serious going on, and needs to be evaluated. in which case you need to take action and try early intervention .
Good luck
M