What age did people start sleep training?
That's like asking "At what age did people start teaching their child?" While it's unrealistic to expect a 4 month old to be doing flashcards and learning colours, that doesn't mean that a 4mo is too young to learn at all. You may well be teaching baby to hold things, pass from hand to hand, learning to reach and roll, and so on.
"Sleep training" is the same as "learning" - there is age inappropriate teaching techniques, and there are age appropriate teaching techniques to help your baby learn to settle to sleep independently.
A dummy , for example, is a sleep training tool. It's a method of "sleep training" parents can use from baby being literally days old. Baby is never too young to learn. You just need age appropriate expectations. Leaving your 4moto cry in the hope that they learn to self comfort is akin to giving your 4mo a Sharpie and being frustrated that they won't draw a smiley face.
But there's lots you can do to nudge your 4mo towards independent sleeping. Doing nothing at all until baby is older, actually, is as unhelpful as forcing sleep training too early. Your baby needs to learn stuff and can be learning stuff to be better at sleeping independently.
What , specifically, is the problem with your baby's daytime naps OP?