I have a 4 month old border collie. She mostly lives in the kitchen which is great apart from one thing. At night when it is dark outside and the lights are on inside she can see her own reflection in the kitchen door windows. This makes her bark at it and run around and generally be very annoying. If we turn the lights off in the kitchen of course the naughty puppy outside goes away!!! However, this means that we would have to eat in the dark.....not very convenient when you have guests!!! We have tried using a water spray but this seems to wind her up even more. Any suggestions?

Put up a curtain on the door. I would favour a close woven muslin rather than net, as it will still allow light through, and not as likely to be damaged by the dog.

Attach at top and bottom using eyes and curtain wire with hooks. You can push it to one side in the day, and pulll across at night.

The obvious answer is: eat in another room from where your dog is.

I think, with careful management, this will in all likely hood, go away.
4 months is very very young, certainly all my dogs at a young age have been curious about their reflections!
Taking Rocky for a walk along a high street when i just got him was an absolute nightmare, cos he was staring and barking at his reflection at every shop window!
Now he just accepts that they are there, and ignores it.

However, i say careful management, as it is entirely possible that your dog could find the way you react to her behaviour in some way rewarding.
If you are doing ANYTHING that might encourage her to do it more, stop! This could even be going to her, or shouting at her to shut up, if that gets her some attention, then she will remember, and do it again!

good luck,
Em