Constant waking up at night at 3.5mo / sleep regression or night-day reversal?

My baby is almost 4 months old. At around 2.5 months he started sleeping 6h stretches at night (dream feed at 11.30pm and he will last till 6-7am). A few weeks ago, around 3 months, he started waking up every 60-90min, crying without any reason (no wet diaper or discomfort), and then asks for a feed 3h later from the