Firstly, I don't wish to enter a discussion about whether or not sleep training is advisable/inadvisable/the work of the devil. I sleep trained DD1 at 5 months (controlled crying, not CIO) and it was the best thing I could have done for us both. I will definitely be doing it again with DD2.
I'm interested to know if anyone sleep trained their baby before 4 months and if so, whether or not you still were hit by the 4-month sleep regression?
DD2 is 3.5mo and I'm thinking that, since I'm going to sleep train anyway, I should just do it now** and in the hope of avoiding it! With DD1, she had to have my nipple in her mouth and be in my arms (no lying down) the whole night and it was horrendous. DD2 currently will only nap 15 minutes without me and so I picture it going the same way. I really don't want to go through it again if I don't have to.
But I won't train DD2 this young (even by my standards, it's young) if it won't make a difference. She'll be my last so I'm (mostly) happy for all the cuddles right now.