The physiology of babys sleep changes at 4 months old. Its a permanent change.
Sleep moves away from the newborn stage, when it was passive like when in the womb. For the first 3-4 months baby's passive state is asleep, waking only when a need isn't met (hunger, comfort, reassurance etc) and then once the need is met, back to the passive state of asleep.
From about 3-4 months old, sleep becomes an active endeavour. You have to help baby to get to sleep, rather than it just passively happening. Also baby starts to sleep in cycles, like an adult does. So there are periods of light sleeping and deep sleeping. It becomes hard for baby to link one sleep cycle and the next.
Sleep tends to go to pot, until you and baby learn new ways to actively get baby to sleep and keep baby asleep. Some tried and tested ways to get babies to sleep, that have been established for years, include
- Sucking (dummy, nipple)
- Movement (rocking in arms, pushchair, bouncy chair etc)
- Feeling secure (being held, cosleeping, swaddle, sleepyhead)
- Full tummy
I wonder if you are assuming this change is to do with daytime/nighttime sleep, when in fact it is just this change in the way baby sleeps.
More sleep = Better sleep.
So the more daytime sleep baby gets (more frequent naps, less awake time between naps), the better quality the babys sleep will be - deeper sleeping, less likely to wake between sleep cycles, easier to get to sleep.
Where as Poor sleep = Worse sleep. An over tired baby is harder to get to sleep, sleeps more lightly so wakes more easily. The overall quality of sleep then declines.
A reasonable approximation for a 4 month old would be 45-75 minutes awake time followed by 30-60 minutes sleep, repeated on a never-ending cycle from getting up until you go to bed (and take baby with you). The awake time will be on the low end if the previous nap was short, with increasing awake time if the previous nap was longer.
A restless night's sleep at this age is most often because of
(a) Not enough total sleep over the previous 24-48 hours
(b) Not enough total calories over the previous 24-48 hours
Both are resolved by looking at the daytime routine - making feeds and/or naps more frequent than they currently are.