All respect for you for being able to function AT ALL on that little sleep.

I have an 8 1/2 month old who had a similar regression at the 4 month mark, and is still now really intermittent and unpredictable with his sleeps, though not as bad as your little one. But it did get that bad at one point - for about 5 nights in a row, and after less than 2 hours sleep at night, DH and I were both at breaking point. I rang Ngala, a WA-only sleep-help line (they also do day stays, night stays and home visits, but initially are available for phone consults.) Within a few minutes, the consultant suggested that DS was eating his dinner far too late - I was holding him off until DH came home at 6/6.30, so that he could feed him, and then bed time was 7ish. She strongly suggested feeding him his dinner at 4/4.30, but no later than 5, and keeping the same bedtime, and it worked straight away - that night he went back to the 'usual' of between 2 and 4 wakenings, but going straight back to sleep after a feed. I found it hard to believe that timing of food could make such a big difference, but it really did, and we make sure he eats no later than 5pm now.

If you're in Melbourne, you can't call Ngala, but I do suggest you contact whatever your local equivalent is, and talk through your circumstances with someone knowledgeable. it won't always entail spending nights away. Maybe it isn't solids timing for your DD, but there might be something equally surprising that you haven't tried (cos you've certainly tried a lot of other stuff!)