If you are employed as a day shift driver and you are asked to start work before 4 am, are you then deemed to be for that day a night shift worker and as such only allowed to work a 10 hour shift… 
Lets see if i can get this right 
I think the answer is yes, unless you have an opt out agreement.
I think the 10 hours refers to actual driving & other work added together but not POA or rest.
NOW I’M GOING TO HIDE IN CASE COFFEEHOLIC SHOOTS ME 
Smee:
If you are employed as a day shift driver and you are asked to start work before 4 am, are you then deemed to be for that day a night shift worker and as such only allowed to work a 10 hour shift…
In the UK if you come under the WTD for mobile workers regulations, and you work any time within the hours of 00:00 to 04:00 then you’re deemed to be a night worker as you said.
If you’re a night worker without an opt out agreement in place for the 10 hour rule you’re allowed to work 10 hours in the 24 hour period starting from the beginning of the shift.
If you work two or more consecutive night shifts the 24 hour periods are calculated from the beginning of the first shift.
The 24 hour period is the important point in night work, even if you had the correct daily rest after a shift where you worked for 10 hours then started another shift before the 24 hour period is up you would find yourself breaking the 10 hour rule.
As ROG said breaks and POA don’t count towards the 10 hours of work time.