The sleep regression hit us at exactly 4 months very hard and our baby didn't come out of it. Plenty of them do apparently after a few weeks, so i wanted to give her time to try to get through it. The chances are that yours will come out of it, so try not to panic,at this stage. But if it goes wrong, here is what we did.
I was determined not to sleep train until she was at least 6 months. I finally broke at 6 months and 10 days, after spending 2.5 months with her waking 8 times a night and with some of her sleep periods only lasting 20 minutes. As an insomniac myself, i couldn't sleep in the tiny windows between her wake ups. I was only able to make it for so long because my partner has a flexible job and works from home so could let me sleep from 6 - 10am.
By this point i was speaking to doctors about post natal depression caused purely by sleep deprivation and they were offering me antidepressants. My partner and i weighed up the potential effects of having a long term clinically depressed and medicated mum versus trying sleep training for a few days. We decided trying sleep training would be less damaging to all long term, provided it showed some results quickly.
We used a method where you check the baby after 2 mins of crying and leave the room after rubbing her belly and talking to her until she stopped crying, which she always did very quickly. Then you return and do the same after 2.5 mins, then 3, and so on up until 5 minutes and then you keep going back after 5 minutes until they fall asleep.
The first night she was asleep in 12 minutes, the second 7 minutes, then 3.5, then 2.5, and then she began to always fall asleep before 2 mins was up. By the end of the week she didn't cry at all, just looked around her room and dropped off. We've had a few hiccups with more recently sleep regressions and developmental stages but nothing even 10% as bad as the 8 wake ups a night. She currently puts herself to sleep and sleeps straight through the night.
Hopefully you won't have to sleep train, but if you do, please remember that it doesn't not always involve hours of crying. Its changed our lives and is the best decision we could have made. But i am glad i waited as long as i did, because she was ready. 4 months is very young and you're baby might well learn to link sleep cycles naturally.