we also went through the same at 4 months. DS was a great night sleeper from 3 weeks waking twice a night only. Around 4 months he went to waking 4-6 times and sometimes being up for 1.5 hours crying. It does pass, hang in there and be consistent. We didn't change anything. DS stayed in his dark room, stayed in his cot, was picked up for cuddles and to calm down, as soon as he was calm, he was put back in his cot and we tried to settle him in there. Was it after his 4 month injections that this all happened?

Good luck, it is so hard. We still have trouble with DS napping during the day and doing one sleep cycle. He is pretty much impossible to resettle after his nap as well.