M.P.
S.H. has explained this situation very well from both your and your son's few. He has regressed, not to get attention, but because he is stressed and having to learn a different way to relate to his different world. Cut him and yourself lots of slack. Keep your expectations for yourself and for him at a low level so that you both feel less stress.
Go back to holding his hand at the street corner. Keep plastic bags out of his reach. You'll be doing this for the new baby soon. Just start it sooner. Don't try to spend as much time with him as you were able to do when he was the only child. Try different ways of spending time with him. Perhaps arrange to share play dates and baby dates with other moms. One mom keep both babies and their other children so that you can literally spend an hour or two with just your oldest son. Have your son watch the baby while you spend an hour with your son and vice versa. I've worked nights as a mom and had the usual amount of time to spend with family as I had working days.
I reread your post. It's not that your husband won't help you. It's that you're alone with both kids at night. Try a different routine. Perhaps your husband could help get your son ready for bed before he leaves for work. He may need to change his sleeping time. The two of you can find a routine that works better for all of you.
Go back to handling temper tantrums in the same way that you did when he was 2 and 3. Ignore them. Perhaps put him in is room and tell him he can come out when he's calm. Before I was a mother one of my friends calmly told her whining 3yo to leave the room until he could use his big boy voice. She said it as if she were saying it's time for lunch. Matter of fact without any irritation. I've also heard her say, I'm getting irritated with what you're doing. Now stop or go to another room with it. Honesty does take some of the sting out of irritation when the parent does not blame the child.
Remember that your son is doing the best that he knows how to handle a very difficult adjustment. You're having difficulty knowing what to do. Think about how little experience this little guy has had and how vulnerable he feels. After all, he does still have to depend on his parents just to exist. Some of his behavior may be testing, although he's not conscious of it, to see if you'll still love him when hes obnoxious.
Instead of spending more time with him, give him more hugs and positive attention with the time that you do have. Deal with meltdowns before they get going to the point of a tantrum. Even if you have to let baby cry for a few minutes, take the time to get down so that you can look into his eyes and he can look into yours and repeat to him what he wants and say, I will ........whatever it is you can do once I get baby's diaper changed or whatever you need to do. "Let's talk about this in 10 minutes often worked with my grandkids even at that age. I showed them where the hand would be on the clock so that they could know when 10 minutes were up. Sometimes I'd have to say another 10 minutes. I got better at estimating how much time I needed.
Hitting means he has a whole lot of anger bottled up inside that has to come out in any way it can. At four he can talk about how he feels. Let him know it's OK to feel anger and help him find other ways to express it. Hitting the sofa or a pillow might help.
Eventually, thru trial and error you will find ways to help him defuse the anger before it reaches the point of hitting anything. One way, that I found, is to say to my grandchild, when I notice the anger, run back and forth in the hall 10 times. The physical exertion reduces the amount of angry energy inside. At other times, I play box with them. I squat down and say, boy you sound so angry. Let's box. I've already taught them that boxing means no touching. We practiced play boxing when they were calm. I immediately stop the play when either one of us gets hit or run into.
When he starts to hit, grab his hand and tell him hitting hurts. Do not hit mommy. and or, We do not hit people in this home. Go hit the couch. Redirect his energy in some way. Then when he stops hitting you, even after he's hit the couch, give him a hug and say you are proud of him for not hitting. You may have to stretch a bit to do this at first. What is important is getting across to him that hitting is not acceptable and that you love him.
What is important is finding a way to consistently show your son compassion for this new life that he didn't ask for. Talk with him. Tell him you understand why he's upset. Do not punish him for being angry. Plan consequences for expressing his anger in a way that hurts. Let him express his anger as long as he isn't hitting you or anyone else. Breaking toys is not acceptable. So if he throws a toy, the toy goes away. That sort of thing.
I sympathize with your whole family. A new baby is a change for everyone. You will find ways to make life run more smoothly. Be open to trying different things. And, knowing that you're sleep deprived and have hormones that may be all over the place, be gentle to yourself. Don't expect to be super mom. Have reasonable expectations for what you can do based upon the realities of this new situation.
A last thought. Cry with your son. I've found crying together to be a great bonding experience.