Some thoughts - without knowing that much about the situation...
The 4 yo may need an assessment but the fact that he's 'a perfect little child' when he's by himself and not when he's with his sister makes me think that maybe he doesn't. I don't know. If you think there's a risk that he is so angry he might seriously hurt his sister then I would definitely get help. Does he go to kindergarten or daycare at all - and if so how does he behave there? Does he behave ok at other people's homes?
Don't let him become the 'bad one' because it will be an identity he'll live up to. Make sure you give him lots of positive attention and I would try and give him some one on one time without his sister - even if it's just 20 minutes of play while she's having a nap. Don't let it become so that the only attention he gets is from fighting with her. Make sure you give praise when he does something right - and give him lots of opportunities to do something right. Give him jobs to do that his sister isn't able to yet - let him be the big boy who helps with the washing up - or something like that.
Try a carrot instead of a stick. For example, 'if you can play quietly until lunchtime without any big voices then we'll go to the park after lunch '- keep reminding them of the deal and praise them a lot when they achieve it. Be matter of fact about it if they don't - don't be angry.
Use natural consequences. For example, if they're being noisy in the car just pull over and don't start the car again until they're quiet. 'I can't drive the car/ get you an ice-cream/etc etc while you are being noisy' Definitely if he can't walk safely to the car you need to hold his hand - that's just dangerous. If you're going somewhere he wants to go the deal is he walks safely holding your hand or you don't go.
Remain the adult in the situation - try to remain calm, unflustered, matter of fact, and don't yell. I really honestly know how hard that can be! But you have to remember that he's a 4 year old.