You're really lucky!!!
the sleep in the first half of the night is usually the deeper sleep, so it's not unusual that ds is more restless in the second half. Dd sleeps in our room too, it's only happened recently that she hasn't woken herself (or us) at some ungodly hour. I like having her in her room, but it does mean that the parent's sleep gets disturbed when it needen't. You just can't help but hear every little thing.
I find it really hard not rushing in and disturbing dd more than is necessary, does he need a new nappy when you change him? A bit wet is ok, if he's leaking or has done a poo then I'd change him, but if not I'd leave it until the morning. 4 months is kind of early to drop the 1 am bottle, but you know your baby better than I do
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Sounds like you and ds are doing great though.