I would say be prepared for what you need to change over time...
4 days and day off is probably great now but you might find 8 - 2 works better in the longer term, allowing you to be more involved at school and similar stuff
Do you have the option of going for 4 days now but switching when school looms? In theory you should be able to change your hours under flexible working so long as there isn't a business reason why not....
The flexible work policy says that I can submit a change once a year, however I'm not sure if that's hours or which days IYSWIM.
My bosses are really great when it comes to flexiblity so I'm sure there would be no problems with switching.
I honestly, don't know which way to go, as there is the possibility of having to travel with my job, having to stay away for a night or two every now and again so don't know how this would effect my working hours either.
It's probably going to sound selfish but I wouldn't want to not be consider for certain projects at work due to them not wanting me to travel if it complicates hours and things.
I would agree 4 days at this stage, as if you are travelling you probably won't get that time back and it is nice to have a full day with your DS. Plus means you will have full days in the office whereas if you are leaving at 2pm every day it may feel like more of a rush.