I get (and I know you get) all kinds of questions about sleep training 4-month olds. "My mother-in-law/pediatrician/dog/dentist's accountant says I should sleep train my 4-month old!"
And it makes sense that people ask that question, because things have been getting progressively better on the sleep front and then it gets bad again at four months. So we think there's a problem and that we can fix it by doing something about it. Indeed, we feel that it's our job to fix it by doing something about it.
No, you should not sleep train your 4-month old.
And here's why: The reason babies go through the 4-month sleep regression or stop sleeping at 4 months or start waking up to cry or just can't fall asleep or whatever is happening with them is that they're going through a normal developmental phase. That phase makes it difficult for them to sleep stretches of more than a few hours. I liken it to the inability you have to sleep when your mind is really working on something (good or bad) so you keep waking up to think about it.
Which means two things:
1. While the night waking may be a problem for YOU, it's actually normal and part of normal growth and development for your baby. Therefore, an issue to be managed in a whole-family sense, not a problem the needs to be solved. You still need sleep, so think about ways to get it even if your baby isn't sleeping through.
2. This is the worst possible time to sleep train because your baby's body is wired not to be sleep trained right now. So you will end up trying and trying and feeling like a loser and wondering what's wrong with you and/or your baby, when it's just bad timing. You are a smart, competent person.
So, yes, you CAN sleep train at 4 months. There's all kinds of stuff that you CAN do. But just because you can doesn't mean you should.
If you wait 2-4 weeks you baby will be through the developmental spurt and will be back to sleeping. If you feel like you want to sleep train at that point, 5-6 months is a far far better time to do it than four months is, because you have a fighting chance of success. And we all want success.
I know there are pediatricians who tell people to sleep train at four months, but remember that most pediatricians don't have any special training in infant sleep. And if they went to medical school before the research on four months came out, they may not know it. So if you're getting pressure from a pediatrician to sleep train at four months, ask them what they think about the research of Granic, Lewis, van de Rijt, and Plooij, and have a conversation. If they can present some compelling evidence for sleep training right then, ok (and please let me know).
As always, if someone offers you advice you don't need (including me), just smile and nod and say, "Thank you for the suggestion!" and then go home and do whatever you want to anyway. You're the best parent for your child.