4m sleep regression is when babies first enter light sleep mode, all humans go through rem sleep every 90m and as we leave rem sleep we go into light sleep mode where we are partially awake most adults don't even recall light sleep when it happens, but when babies experience it for the first time they wake up all the way. Should only last about 6 weeks

A sleep regression is when a baby falls back into his newborn sleeping habits, or lack thereof. They have them during growth spurts and there's one that commonly occurs around 4 months of age. That one coincides with a period of rapid mental development where baby is beginning to figure out many patterns and sequences.