Ha, oh mama. Our entire DDC had a total breakdown around the 4 month mark. Our 4 month regression was particularly pronounced for whatever reason. My daughter went from sleeping 5-6 hour stretches at night to waking every 2 hours for at least a few weeks. It was totally exhausting. Napping was terrible too. It would take me an hour to get her down for a 30 minute nap. Argh!
We had a lot of success with the No Cry Sleep Solution. The Pantley Pull Off really helped my daughter learn to self-soothe at around 5 months. She gradually cut out night feedings/wakings from there. We never let her cry, but I did have to learn the difference between sleepy fussing and crying. She would sleepy fuss for a few minutes before grabbing her lovey and falling asleep.
A few things that worked for us: get a lovey if you don't have one already. My daughter now grabs hers, stuffs it into her face, and falls asleep. In the beginning she didn't seem to pay any attention to it, but it's definitely soothing for her now. Establishing a routine helped - we usually do diaper change, story, nurse, and bed. Also, my doctor gave us great advice when she was a newborn: unless it's poop, no nighttime diaper changes. I got a heavy duty overnight cloth diaper system so she could be fed quickly and placed back in bed during night wakings. I think this helped to establish good sleep habits at the beginning.
For really awful nights, we co-slept. We don't co-sleep anymore because I don't get sleep and she doesn't either, but on those nights when she was up every 1.5-2 hours, bringing her to bed was the only way to get any sleep at all.
It will pass. I love the Pantley book because it at least gives you hope that things will get better. In our case they got so much better by 6 months I couldn't believe it. But having a plan, no matter what it is, can help you get through the worst of it. Now we're in teething and 9 month regression territory, so I hope this one passes soon too. Good luck, mama!