So how long does a 4 mth sleep regression last?!
Hugs to you, mama! My DD is 11 months, and we are still in the 4-month sleep regression. :( It seems like, for us, starting around 5 months there have been NONSTOP legitimate sleep challenges - teething, gas pain from solids, developmental milestones, etc., one blending into the other. But some nights are better than others, and cosleeping is what is saving our sanity! I'm exhausted enough as it is, but when I was trying to sit up and nurse, then get baby back down in a crib, I was ready to jump off a bridge.
My suggestion to you is to claim some rest for yourself however you have to! Expect a break, demand a break. Take care of YOU. This is NOT your fault! And check out the "No Cry Sleep Solution" - for us, so far, the strategies haven't really improved much, but I at least feel like we're building some good patterns that will come in handy when she does finally get past these hurdles.
And we also sidecarred the crib - we have a queen bed, and it was tight with all 3 of us - babe slept with her face right at my boob, so naturally she wanted it in her mouth! With the sidecar, I can scoot over to baby and nurse, then scoot back to my own space. I'm gradually noticing her getting more comfortable sleeping by herself now - she still nurses a lot and cuddles plenty, but occasionally she'll roll over in her own space, stretch, and actually go back to sleep! Woohoo!
Good luck to you!