This is just my two cents, but I want to put it out there in case it helps anyone.
I've said 1,000 times on this board and others that I am anti-CIO. I've also suggested that people not conduct ANY kind of sleep training during the fourth trimester (months 1-3 of life). Now that many of our babies are coming up on 4 months, I know a lot of moms will be ready to sleep train.
There is nothing wrong with doing so. You aren't a bad mother bc you want to sleep and heaven knows your baby needs sleep!!
But you have to know your baby. You know your baby best, and only you can determine what sorts of sleep methods will work for your family. My first baby (Jack) was high needs. He had extreme separation anxiety starting at 12 weeks. And, while some kids can cry a little bit to burn off some steam and then they feel better, Jack was the type of baby who would cry, and cry some more, and the more he cried the more it escalated. For all of these reasons, CIO or controlled crying would NEVER have worked for him. There was just no way. And I'm not the type of mom who can handle CIO, my ethical issues with it put aside. However, not all babies are like Jack. Rhys is very different and I'm sure that our methods for helping him learn to sleep well will be different.
The basic point of this ridiculously long word-vomit post is that sleep training is not automatically evil and horrible. There is a baby out there and a family out there who will be a model success story for every sleep method on the bookshelf. You have to know yourself and your baby, and you have to know when everyone in the family is ready. Then, and only then, will ANY sleep training of any kind work. If you want to do Ferber at 4 months, and you feel that it is right for your baby's personality type, then good luck and God speed. Don't beat yourself up about it. What I do for my baby, and what anyone else on The Bump does with her baby, has nothing to do with you and your baby.