for everyone suffering the 4 month sleep regression. It hit us hard, DS slept well from birth just waking up for one feed at about 3am then all of a sudden we were up hourly through the night. It lasted from about 3 months to 7 months with varying degrees of severity.
What sorted it was sleep training - gradual increments of crying, so 3 mins, 5 mins, 7 mins etc, going in for 2 minutes to settle each time. We got to 10 minutes once and the next night he slept through. He's been a fantastic sleeper ever since, goes down like a dream for naps and bedtime, sleeps at least 12 hours. Of course we've had the odd blip with teething, illness etc, but otherwise all good.
It's hell at the time but it will end. I really focussed on getting him to self settle for naps because I could deal with it during the day whereas I gave up in the middle of the night when it was dark and everyone else was asleep. If they can learn to self settle then you've won half the battle - I think that's why it took such little sleep training with DS because he had the ability to self settle but had just got into the habit of needing us at night time. Once we'd broken that habit, hey presto. The penny also dropped for me when I realised that self settling was something he had to learn to do, just like sitting, crawling etc. You wouldn't sit a baby up without expecting them to topple over sometimes, or just crawl straight away without falling on their tummy and getting stuck. Unfortunately this means a little bit of crying whilst your DC works out that they can fall asleep without the help of a parent, but I think if you can teach them the art of self settling for naps you can minimise the crying to a great degree.
I hated letting him cry and really appreciate that it's not for everyone, so don't want to imply that that's the only way of solving it, but that's the only thing that worked for us and maybe someone else will be along with an alternative method that worked for them. I noticed a massive improvement on my DS's mood during the day, as he went from being happy and content before the sleep regression to grouchy and whiney when he wasn't sleeping properly. As soon as he got a decent night's sleep he was back on form, smiley and content, and I used that to remind myself that I'd done the right think by letting him cry just a little bit.
Good luck everyone!