You sound as though you're at the end of your tether, and I really feel for you,
but you really can't take the risk of leaving your 2 boys together.
Not whilst the 4yo is hurting the baby.
Do you have any lockable rooms to leave baby in, whilst it in the toilet, etc, as a last resort if you really can't take one of them with you?
I volunteered with HomeStart for 7 yrs. They're a fab charity, doing great work, but underfunded in many areas, so although I'm pretty sure you can self refer (or ask GP ir HV to refer you), they may have a waiting list.
It's essential to have this support put in place as soon as possible, so start by talking to your GP, tell him what you've said here, and how much you're struggling to cope.
Good luck, and don't forget, you've taken the 1st step already, by admitting that there's a problem. That's often the hardest thing to do.
One mum I supported, did actually just need time for herself and I spent time with the children whilst she had a bath, or got on with some other things. It really helped her to have a few hours 2x a week to herself, as she also had no other support.
I hope things work out for you, but please get it sorted asap. Speak to someone today. 