The truth about the four month sleep regression

Somewhere between about 3 and 5 months, babies have got their days and nights sorted out and will have settled into a mature pattern of sleep cycles – with light sleep, deep sleep and waking phases. 
They will also now be producing their own supply of sleep hormones, [melatonin] and they may also have the body weight to enable them to sleep for 5 hour or more stretches at night without getting hungry. 
So given that all this positive stuff is going on, it can be frustrating when babies start to go backwards with their sleeping. Why does this happen?
Because also between 3 and 5 months, they become more conscious of what is going on and they start to make learning connections.
These connections, especially if they are made at sleep onset, can develop into expectations and preferences around their night sleeping and napping. So as an example – if they are fed or cuddled to sleep at the start of the night, they may need those same sleep triggers” when they wake up naturally later. 
Of course it is fine to cuddle or feed your baby back to sleep if that’s what feels right for you or if that’s what they need, but bear in mind that if at this age, your baby starts to wake for feeds and cuddles than they used to; there is a good chance that they are doing out it of habit rather than need, and this can be really disruptive to their sleep and to yours also.
How to deal with it.
A good bed time routine really comes into its own now. You need a repeated series of steps leading up to bed time that baby will recognise as sleep cues.
Then unless youve chosen to co sleep, its good to help them fall asleep in the cot aware of where they are. A lovely way of doing this is to give the last feed with the light on and follow it with a little book. Then lights off and put them down into the cot awake. Soothe them there if they dont like to be left, or leave them to self settle if theyre ok with that.
As sleep now happens in cycles; it is normal to wake a few times in their sleep. When your baby wakes up, they will need to have everything around them, the same as it was when they fell asleep. 
So the more independently they fall asleep at the start of sleep, the more likely they are to be able to join their sleep cycles by themselves.
If they do wake up, try soothing them in the cot for a while rather than automatically picking up or feeding. They might just resettle and drift back off, but of course, if they continue to cry, it is best to respond with whatever feeding or cuddling they need.