Hi,
Hang in there, I know it's painful to see your little one suffering. But I think you hit the nail on the head -- a lot of it is his age. Naturally, sibling rivalry will play a part, too, but from what you're saying it seems to only be exacerbating existing tendencies.
My youngest is 5 now, and I've survived three others passing through those ages. Age 4 can be... interesting, to say the least. The bursts of defiance, the roller-coaster emotions, the eruption of soul-wrenching sobs because he wanted triangle sandwiches, not squares -- yup, sounds about right. 4 is rife with jealousy, too -- this is about the time kids start throwing around the ' he stole my friend' line. With my girls, this is when I witnessed the first knee-high cliques forming (grrrr...). With my boys I noticed increased aggression. Some of my kids' friends went into testosterone overdrive -- hitting, face-screaming, chest-pounding, foot stamping -- thank heavens my boys were a bit more mellow. But the whole 'he-man' thing seems to crop up a lot at this age. And around age 8, as well (heads up)!
I suggest two things (I'd do it for you but I'm in a time crunch tonight):
Google "child development age 4 aggression" or some similar thing, and you'll see a bunch of results that will tell you your son's right on track developmentally -- but you should also see that it's peaking -- meaning it should ease off within the year? Here's one page that's pretty helpful:
http://resources.atcmhmr.com/poc/view_doc.php?type=doc&am...
(Don't be alarmed or offended, the page addresses normal child development information, but it's on a site which also deals with mental health and retardation. The info applies to the average kid, though, honest.) :-)
I'm sure new-sibling syndrome will be in there somewhere. My kids were very close together, so they were too little or too shell-shocked from their own births to really show much jealousy. I'd say your best bet is loving affirmation, inclusion and routine blended with a determination not to coddle him (because kids may not be able to spell Power but man, can they wield it)!
The other thing I'd Google is Supernanny. I know a lot of people laugh it off, but seriously, those techniques saved my life. Really simple, really effective, really free -- just remember to be really consistent.
Hang in there -- the proof that you're doing a great job is that you're humble enough to ask for new ideas! With that kind of open mind, you'll be just fine! :-)