Losing my mind - all transfers failing!

Hi all, I suspect I know the answer here is the dreaded 4-month sleep regression, but looking for any solidarity, tips or insights anyone might have.

My nearly 19 week old has this week started waking on every. single. transfer. to her next-to-me. And I mean every single one. The last three nights her dad and I have slept in 3-hour stints taking it in turns to sit up and hold her for sleep and I am already over this arrangement. It also won't be sustainable as he has to get up for work.

I expected more night waking and more disruption, but never expected this. What on earth can we do?! Our health visitor does not recommend sleep training. I'm not even sure I really know 100% what is sleep training and what isn't anymore.

I have tried transferring from different positions, different lengths of time, warming the mattress, practicing self-soothing (until she cries), sheet that smells like me... running out of ideas and patience here. Help!