Hi,
I have just returned from a camping holiday in Dorset - a 6hour plus drive away. I also didn't want to have to stop at every service station so loaded with water from early morning and then stopped 2 hours before the journey. Then I just have sips for the drive (got about 500mls down) still had to stop twice though. Felt a bit rubbish when I got there but then downed loads because I was so thirsty. Thought I would be fine then - but then the rest of the week the weather was so rubbish that I didn't want to have to leave the tent to go to the toilet block so barely drank 2 litres a day and hardly any at night. Result - lowest weightloss so far this week and felt very hungry for a couple of days afterwards.
I think if it is just the journey that will be an issue - then just try to make up the water when you arrive (you will prob be thirsty enough by then to down a good litre or so). It is not ideal as you are supposed to drip feed your system with water to keep it ticking over - not starve it of water and then drown it - but if the alternative is not getting through your full 4 litres - and it affecting your weightloss - I think it is the lesser evil.
Hope this helps