It's so unpredictable though
The physiology of sleep changes at around 3 or 4 months old. So it is not unusual that nothing that worked in the newborn stage continues to work. The "fourth trimester" (ie the first 13 weeks or so of baby's life) is biologically very unique.
In the first 3 or 4 months of baby's life sleep is passive. It mirrors the way things were in the womb when all needs are met baby will sleep. Then will only wake when a need isn't met (hungry, uncomfortable from a wet nappy, pain from a bowel movement etc) and once that need is met baby will sleep - just like when in your womb.
Baby doesn't need to work at being asleep because it is a passive, no-thought-required state of being. As long as baby has no alternate need, baby will sleep and awake times are brief in the fourth trimester.
From 3 or 4 months old, baby's sleep matures and develops into cycles. Much like adult sleep cycles, sleep will have periods of deep sleep, lighter sleep and brief "environment check" semi-wakes between one sleep cycle and the next. Baby wasn't sleeping in cycles before.
Now is when sleep stops being passive and becomes active. "Getting to sleep" becomes an active process that baby (and by extension parents) have to work at achieving. It is no longer something that 'just happens' as it did in the passive fourth trimester phase. The work needed to get to sleep is also used at those environment check brief wakes between sleep cycles (often the cause of poor sleep in older babies).
It is at this age - 3 or 4 months old - that parents need to start developing sleep associations that will last a long time. Those lovely, easy, long naps of the newborn days become harder. They will come back when baby learns ways to link sleep cycles and go back to sleep when briefly waking. But that does not mean this is a "regression". It isn't. Likewise feeding to sleep is easy in the newborn phase when sleep is passive. Not so when sleep becomes active. That said feeding to sleep can work again (for a time anyway, not forever), but in the context of the matured sleep.
I read lots about people saying the 4 month sleep "regression". It isn't a regression.
A regression suggests that if you do nothing then everything will go back to how it was eventually. This isn't the case. The actual biology of sleep has physically changed and so baby and parents have to change to deal with this change. Understanding the physiology helps understand what is happening.
The quicker parents and babies adapt, the shorter the "regression" (that isn't really a regression) lasts. Some families have sleep habits in place in the fourth trimester that translate smoothly into mature sleep and so they never notice a "regression" at all. Other families cannot easily find ways to adapt to the new mature sleep and so for then the "regression" lasts longer.
I don't know if that is helpful to you or not? I found that understanding the science of the whole sleep situation helped me to understand what was happening better.