Hi Bee72, I wasn't on the original thread but, like you, I dug it up to look at during DD's 4-month regression!
She also started at 3 months and really it went on until she was about 19/20 weeks. At the time, I hated hearing things like 'it will pass' etc. etc., I simply did not believe that it ever would. She was (still is) breastfed, and she was needy and waking constantly throughout the night because she couldn't self-settle.
Time helped - she got a bit better on her own, I remember the exact night it happened because DP and I were at my parents'; I put her down to bed, and went back to the kitchen, and DP and I WERE ABLE TO EAT DINNER WITH MY PARENTS. As in a full, long, dinner! She did continue to wake lots for several weeks after that but once I sensed she was improving I tried to encourage it in different ways.
I taught her to self-settle by using the pram. She was good at going off to sleep in the pram when we were out walking, so I figured she could do it in the kitchen, too. I zipped her into her snowsuit (it was winter), and basically threw every sleep prop available at her apart from the boob. White noise, comforter, 'trigger sleep'music, and vigorous rocking. She'd whinge a bit and eventually drop off. Once she was dropping off without whinging, I started to take away one sleep prop at a time. First I took away the trigger music, and then I stopped rocking the pram as soon as she was asleep. And then I stopped rocking the pram at all - just put her in pram and she went off by herself.
Then I transferred her to the cot and zipped her into grobag to recreate the snowsuit effect :) In cot with trigger music, white noise and comforter. She took to this better than I could have hoped, and soon I didn't have to bother with the trigger music. The white noise is handy, though, I think it makes them sleep for longer.
I'm not sure if any of this helps. Top tip is to introduce a comforter, esp. if you're breastfeeding, and create an identical routine for sleep. It'll take a while to kick in but it will eventually.
Haven't yet had to resort to CC or CIO, thankfully, but she's only 6 months, she could regress again!