Steevo25:
So my question is if I needed to drive and then reached the 4.5 hours, if I had another 45 minute break, could I then still use some or all of the 2.5 hours I lost from the first 4.5 hours when the tacho went back to zero?
You never lose driving time
In your case you can do 2 then 45 break then 4.5 then 45 break then 2.5 on a 9 hour drive day or 3.5 on a 10 hour drive day
The daft thing is that the more driving time breaks you have the longer the work shift becomes