Check the carotid pulse for 5-10 seconds. If you feel no pulse within 10 seconds, start CPR cycles of 30 chest compressions then 2 breaths. Compress the center of the chest hard and fast (atleast 2 acheiving at least 100 compressions per minute) Allow for full chest recoil after each compression.
Carriers are required to provide adequate food and potable drinking water for passengers within two hours of the aircraft being delayed on the tarmac and to maintain operable lavatories and, if necessary, provide medical attention.
We stop half way for about 15 minutes. As long as it's not every day, I think you'd probably be fine doing the 4 hours in one go really, but I try to think how comfortable I'd be for that long in a baby seat, cos they can't move as much as us in the car. Just long enough to stretch and wriggle about a bit is long enough for a break I think.