Hi,
My daughter is bottle-fed and like you I keep hearing and reading everywhere that bottles should be made up one at a time. Nice idea but totally impractical when you have got a screaming baby.
I live in an area with hard water and as far as I am aware as long as you boil the water first it is fine to use, I don't think you are supposed to use filtered or bottled water so there is no other choice. I also make up four bottles at a time and store in the fridge and as long as they are used within 24 hours they should be okay. I don't know about putting just the water in the fridge and adding the powder when needed as like you I was under the impression that the water must boil and then powder must be added 30 minutes later when the water is still around 70 degrees. I read this week that this is because the milk powder is not sterile and at this temperature it is more likely to kill off anything!
When I was pregnant my midwife told me about this new recommendation to make up a feed at a time because she was required to but even she said it could be risky as in the middle of the night when sleep deprived measurements of milk could be got wrong and that is potentially dangerous to baby.
My daughter is now 12 weeks and I have been doing the bottles this way since she was born, she is perfectly healthy and happy. I also have ten year old twins and did the same for them and there is nothing wrong with them either, so personally I will continue this way. As I say I think if the water is boiled and bottles made up 30 minutes later and then put straight into the fridge which is at the correct temperature it is okay. (I also heat the bottles in the microwave and shake well - how naughty am I!) If I am going out I do put an ice-pack in the bottle cooler bag and if I have taken two bottles and have not used one within 4 hours I throw that one away to be on the safe side).
You do what you think is right for you and your baby. Personally I think some of this could be an attempt by the health officials to try and put people off bottle feeding and promote breast feeding but that is not always possible as I have found out.