Yes i do agree with aviatrix that taking a toy from him so he can see how his little brother felt, will just make him feel that you are being mean to him; I think the lesson is lost, all he experiences is mum taking a toy to make him feel bad! You don't need to work that hard at it in my view. Talking IS enough. He needs to be told it's wrong, and why; and he needs to see you give good sympathy and attention to ds2 as the victim; that is all you can do in my view.
i think you're trying too hard to deal with it and make it go away (totally understandably) but I think it is better just to accept that it is a long process and your older boy is still very very young and only just learning the social rules.
I think also take every opportunity to build their fondness for eachother, tell one how much the other loves him, or looks up to him or admires how he can do this or that etc etc etc. It will eventually pay dividends I think.
And yes, your four year old is a normal 4 yr old, scary as it may seem when they are being a nightmare, but it is normal to have little regard for other's feelings; he's not really being malicious, just acting on the spur of the moment and being unable to fully govern his feelings and actions yet.
i say have confidence, you will get there if you persist in your approach; and FWIW I don't think there's anything wrong with being sent to have a think about what he's done. It doesn't have to be like a 'punishment' as in supernanny "You Will Sit There For 4 Minutes By The Timer", more genuinely a time for him to have a break and think about it. My mum occasionally said we should go and write a story about what had happened and that worked really well in helping us to work out why we'd been angry and get it all out.