4.5month... I think we were aiming for 7-8pm then. DS was a shocking sleeper at this age (due to pain) so most nights he was down for 8-9pm-ish. We didn't let him have a nap after 4pm then - we wouldn't wake him if asleep, but if awake he wasn't going to have a sleep.

It's reasonable and I would make it easier on the whole family by moving his bedtime by half an hour a week - he'll be too surprised if you move it three hours all of a sudden.

HTH.