Consider that with all the adjustments this too shall pass.
Keep consistent in your disipline method and try not to give your 4 year old too much attention for the bad behavior. I always try to remind myself that whatever I focus on will expand. Some bad behavior needs to be ignored especially if he appears to be doing it for attention. When he behaves badly and it is obvious to you that it is for attention, tell him, "I see that you want my/daddy's attention that we're not paying attention to you, but that's not the way to get our attention. We're busy with (you fill in the blank) and when we're finished well give you our attention." --This works with my 3 year old.--
With the agressive behavior toward the 2 year old, remind the 4 year old constantly when he nears the sib to "use his kind and gentle hands." Show him by taking hold of his hand what that means and touching the sib gently. Remember you will have to do this over and over and over. Also tell him that if he does not use his gentle hands the consequence will be an immediate time out.
Remember to tell your husband whatever it is you decide to do, so that he follows through too.
Try to get your son in bed at the same time nightly. I know it's hard, it is a challenge in my home, especially at summertime. Is your son still taking naps? If he's not, be sure he still has some down time. This may mean 1/2 hour of quiet time in his bedroom reading (looking at books) or playing quietly, but he has to stay in his room.
Lastly, I always think about what I've fed my child before he starts acting wild. Has he had enough fluids, protein? Does he maybe have a food alergy? Some kids react differently than expected with food alergies. It is certainly something to consider.
Good luck,