It can help to know what’s going on at 4 months.
When your baby is a newborn, their default position once they have fallen asleep is to stay asleep until a biological imperative (hunger, cold, wet, etc) wakes them. This means you often get a golden period of sleep between about 3 and 4 months, where your baby can go long stretches without feeling hungry, and therefore stays asleep for hours at a time. This was exactly my experience - from 10 weeks to 20 weeks my baby would sleep from 9pm to 7am without waking.
At around 4 months your baby’s sleep matures into the pattern we still have as adults - sleep cycles which move between deep sleep and light sleep / wakefulness. In the periods of light sleep / wakefulness, your baby may wake fully and need resettled. This is a normal part of human sleep - as adults we wake between sleep cycles, but we usually fall back asleep without even realising we have woken.
What makes the difference to your sleep as a parent is whether or not your baby can get themselves back to sleep between cycles, or whether they need help. If they need help every time, you will experience frequent night wakings.
There are things you can do to help - the main one is helping your baby get used to falling asleep without intervention by you in their cot. However - not all babies can manage this. Mine was utterly inconsolable if put in the cot awake. He’s now 7 months and is only just beginning to be able to do this. If you have a baby who simply won’t tolerate being placed in a cot while awake, it’s very hard for them to link sleep cycles.
Alternatives you might consider are co-sleeping (see the lullaby trust on how to do this safely) or transferring your baby to the cot once asleep and accepting they’ll need your help to resettle.
Lots of sleep training is about helping your baby learn to ‘self soothe’. This is a bit of a myth. Babies can self-soothe - some babies will happily hum / babble / drift off to sleep themselves. But a baby who is distressed or disregulated can’t soothe themselves. So if you have a baby who is distressed in their crib, you can’t teach them to self-soothe by leaving them to cry etc. You can only teach them to shut down. A distressed baby needs intervention from their caregiver to be soothed. Babies learning to fall asleep on their own is largely a developmental milestone; you can’t force them before they’re ready.
That said, sleep training to an extent is possible from 6 months (as long as you have realistic expectations of what this means). I personally really like Lucy Wolfe’s book ‘The Baby Sleep Solution’, because it doesn’t ever suggest leaving your baby to cry. Instead, you stay with your baby and soothe them to sleep in their crib. This has really helped us with my baby.
So, in short - all babies go through the developmental leap of a maturing sleep cycle at around 4 months, but it won’t necessarily lead to worse sleep if your baby is capable of self soothing. If your baby can’t self soothe (which is very normal and common) you will likely experience night wakings. Having realistic expectations of baby sleep and support for night wakings can help you deal with this. Sleep training is possible to an extent, but any method involving leaving your baby to cry is essentially teaching them to shut down, not to self soothe.