www.mumsnet.com/babies/child-development/3-month-old-baby - "Her sleep pattern should be much more reliable by now, with her sleeping through large chunks of the night."
www.mumsnet.com/babies/child-development/4-month-old-baby - "Your four-month-old may well be sleeping much better now, with a good long stretch at night time, and staying awake for longer periods during the day."
There is some decent advice on sleep regressions and how to survive them on the Sleep pages - www.mumsnet.com/babies/sleep#sleep_regression. But the baby development calendar doesn't mention it and would have you believe everyone else's baby is sleeping like that smug mum at the baby group who lectures you on her "bedtime routine from Day 1!" any time you yawn.
So. Given how common a 4-month regression can be, and given the utter has-my-baby-been-possessed-by-a-demon? hell it is, is it worth linking to the existing sleep regression section from the baby development calendar?