I'm going to ask you a question.
Could you live with yourself if your child became one of those statistics and you hadn't followed the guidance?
I know where the guidance comes from. It comes from research, some of it from babies who died from SIDS, some from babies like my DD, who, through having paternal family history of SIDS (her paternal aunt, and her father's cousin both died) was monitored on the Care of Next Infant programme from the Lullaby Trust. As part of that we were asked if we would participate in research to help prevent SIDS, and we said of course. We recorded every feed, every nappy, weighed her daily, and she slept wired up to a heart rate, pulse, and oxygen saturation monitor until she was ten months old.
She also had an Angelcare monitor.
At five months old, at 2.30am one Dunday night, my dd stopped breathing. Her consultant paediatrician issued monitor went off and woke us up. Her cot was next to me, and lifting her started her breathing again. The shock made her scream the house down.
Her Angelcare monitor didn't go off. She wasn't breathing, but there was enough movement in her body that it didn't trigger until I lifted her off it. Even though her body was moving enough to not trigger the monitor she wasn't getting any oxygen.
I dread to think what would have happened if she hadn't had the hospital monitor, or if she hadn't been in our room.
The guidance exists because of the research done on babies like her aunt, and babies like her uncle who was born and studied after his sibling died, and dd who was also studied. Those families relived the worst thing anyone can so that researchers could learn from the deaths of their babies, and those families who have every minute tracked and monitored, all to try to prevent anyone else going through the same.
I think people forget that the guidance is based on research into thousands of dead babies. Thousands of families, who just put their baby down to sleep and they never woke up.
So, to answer your question - yes, a baby CAN move into their own room at 4 months, but why on earth would you risk it?
If I had, my dd may well not be here. And risking that just for a good night's sleep? There's no reason good enough. Your good night's sleep could turn into the last good night you ever have. My PIL have never recovered.