HELP! I don't how much more I can take!!

My DS will be 4 in Febraury.  He has never been a good sleeper.  I always knew this, but now that I have my DD who is three months, it is even harder to deal with.  He frequently wakes up screaming and crying and can't really tell us what is going on.  Sometimes he needs to go to the bathroom, but sometimes he doesn't need anything.  My three month old is sleeping all night and has been for some time.  I know God knew I could handle another one that doesn't sleep!! LOL!  My husband has been out of town all week and my DS has had a really bad week as far as sleeping goes.  He was sick last weekend and has missed daddy this week, but he has been waking up several times each night screaming.  Last night it woke up the baby because he was so loud.  I feel like we have tried everything.  We sometimes give in and let him come in our bed, but things are no better.  My fear is that he will continue to wake the baby and mess up her great sleep habits as well.  I have talked to two different peds. about it when he was younger, but their suggestions don't help.. Do any of you have any ideas?  I lay there at night feeling so mad and frustrated that my 3 month old can sleep all night and my almost 4 year old can't sleep more than a couple of hours at a time.  I need some help!!

[Edpsymom]

I know you have probably explored these options, but for what it is worth....
I would think of: snoring (some kind of obstruction in nasal area or sinuses),  night terrors,  pain,  separation anxiety,  pinworms,  or diabetes.  

Good luck...it is difficult on you as a parent to continually lose sleep, but it is hard on the child also. Try to establish regular sleep patterns with calming pre-bedtime routines, limit daytime napping, and every so often, get grandma to keep the kids overnight so you and hubby can sleep.