Meant to say, I really identify with the feeling on-duty 24/7.
My advice is:
Let the housework slide. Only do essential jobs during day. Save big jobs for weekend and share them with DH. Getting out is more important than having a spotless house!
Get out of house daily. It's good for baby to get out and see things, watch you socialise etc. They get bored being indoors all day. Baby classes aren't leisure time, they provide stimulation and entertainment for baby. Try sensory classes, swimming, music, storytime, buggy-fit etc. Go for coffee/lunch with the mums afterwards and don't feel guilty- you need a social network (and they may become playmates for baby later!)
Build up a network of mum friends and meet regularly. I often go to friends houses and we take turns to watch babies while others take a break, make coffee, bake, get hair done etc (mobile hairdresser).
That allows me to build 'me' time into the day rather than waiting for DH to come home. Also I take advantage of DS's naps to take some leisure time- if he naps on me I read my kindle or MN or play games on my phone.
Re breastfeeding, it's not normal for a 4-month baby to feed this much. Get help urgently (lactation advisor/GP/BF group/HV). He is probably comfort sucking. Or you may have low supply and he is feeding excessively to try and increase it.
Also try expressing milk and let DH feed him on eve so you can take some time off. This will also help if he has a poor latch or struggles feeding from breast.
Try going out for a few hours on a weekend so DH has to cope without you. He'll figure out how to soothe baby better than if you keep coming to help him! And you get a proper head-break!
Also try to divide up evenings and weekends so you both have leisure time. So if he gets home at 6pm, you could have a meal ready then hand over baby so you get some time off to have a bath, read a book, ring a friend etc. Then you could take baby back for bedtime routine around 9pm so your DH can take a couple of hours to himself too. This is what we do.
I find caring for a baby the hardest 'job' I've ever had. It's physically and mentally draining, my head is foggy with sleep deprivation, my back and knees ache so much I often can't sleep. I'm frequently in tears by time DH gets home after hours of rocking a crying refluxy baby. But it's also the best thing I've ever done. I get all the best bits, the little special moments DH misses when he's at work. Sometimes he tunes in on the baby monitor just to watch his son sleeping. He feels he misses out when he's at work so I try to make him feel included and central when he's home.
It's importabt to keep communicating with your DH. If he's keen for you to do less housework and get out to baby groups, try it! He will feel happier just knowing you are happy and enjoying your day.