Singapore are great and don't even worry about doing the washing out or anything. Just call them over and explain that you need it steralised and ask if they can wash it out with boiled water and maybe soak it if they can. They'll be happy to help.

I know the stewards/esses like to be busy and they'd rather help you out so you can look after the bub then you having to do it

GL, you'll be fine. Four months is be a great age to travel - they won't get bored, don't have to occupy them, don't have to do laps lol! Just wait till bubs in walking then the plane fun starts!!

Are you going by yourself or is there two of you?