I'm not sure it's fair to say that self settling is not possible until older, a bit of a sweeping statement. I think it massively depends on the baby.
Some babies are not ready and need help to sleep and that's absolutely fine. However my DS started settling himself to sleep to some extent with his dummy from 8 weeks. Then we hit the 4 month sleep regression (eep!) and ditched the dummy, so there was a few weeks adjustment but he's been settling himself fairly consistently since 15 weeks or so. In fact, I can't settle him any more! Rocking, patting, shushing etc all seem to annoy him. Just chuck him in the cot with a full belly and his muslin to cuddle, and he generally drifts off in minutes. He can also settle himself back to sleep when he stirs between cycles.
This isn't a brag. His sleep isn't perfect, he still wakes at least twice a night to feed, often more. Sometimes he gets up for the day at 5am. Sometimes he needs a bit of help, like being played a particular song he likes etc. We don't believe in leaving him to cry and always intervene if he's not settling (if so, there's normally a reason such as still hungry, wind, dirty nappy).
Anyway, just mentioning it as I've heard that phrase used many times, and while it might be correct that self settling is developmental, I'm not sure I believe that all babies need to be older for this to happen. Certainly some babies won't be ready, but others will and unless given a chance, how will their parents know if they can do it?
I actually discovered accidentally that my DS prefers to be left alone to sort himself out. He was crying one day and when nothing I did helped, I put him down out of exasperation as I'd run out of things to try, then he pretty much rolled over and put himself to sleep! After that, I gave him a little space to work things through and found that actually he was capable of settling himself but I assumed he couldn't and kept interfering, actually making matters worse.
I'm just mentioning it so that people don't automatically assume that no baby at 4 months old can settle itself. Some can, mine can, and we didn't do anything harsh to make it happen.