I see that you have already accepted an answer but I wanted to provide a different perspective if you (or another parent who happen on this question) decide that sleep training or a cry-it-out method is not working for you.
As a first thing, as you probably already know, 4 months is a specific landmark in development where the sleep pattern of a baby change and become more adult-like:
3-4 months. Baby sleep patterns are becoming more adult-like. Infants no longer plunge directly into REM after falling asleep, and their sleep cycles begin to include longer stretches of slow-wave, "deep" sleep (Schechtman et al 1994).
This usually means that the baby will cycle through stages of lighter and deeper sleep, thus increasing night wakings, but it may also mean that the baby will have a harder time falling asleep, just like you are experiencing.
We went through the very same experience as you, around the 4 months mark all the things that used to work didn't work anymore (an the yoga ball was a last resort for us too!)
We didn't try Ferber or another cry-it-out (CIO) methods because my husband and I agreed that it was not for us.
I will offer our experience with the caveats that
- non-CIO methods like we used are probably less effective than Ferber et al.
- they require lots of time and patience
Overall in the end we are happy with how things went, now my son is 1yo and putting him to sleep in the evening is generally a painless affair dealt with in 10 to 30mins. He usually falls asleep on his own, laying down quietly with me or his dad by his side.
What we did after the 4 month sleep regression hit was introducing a schedule and a bedtime routine.
Trying to have a regular schedule during the day is important for two reasons:
- Regularity, consistency and knowing what is going to happen helps babies keep calm
- You want to make sure that your baby sleeps enough and does not arrive at naptime or bedtime too tired, because then she will have a hard time calming down enough to fall asleep
Again, there are two reasons why this is important:
- Same as 1 above: once the routine is established the baby learns that this particular succession of things leads to sleep and this will help her arrive at the sleep moment more calm and relaxed
- The routine itself should contain elements that you know are effective in soothing your baby. You mention that the old tricks don't work anymore but as your baby grows you can try new ones :) for example, there has always been a music moment in our routine but the kind of music changed over the months from hits of the nineties to Mozart
The most important thing
What I think is the most important thing you can do is to try to understand your child and the trouble she's having. I know it's hard! I fought a lot with my husband at the time and I couldn't help being angry at the child too sometimes.
But really your baby needs sleep, she wants to sleep and she cries because she feels she's too winded up and can't get to sleep. That's the simple truth. So if you can help her calm down and settle she will sleep.