On private leases the deals are usually better over 4 years rather than 3 both in terms of AP and rental per month.
Depends on the vehicle. A Dacia is almost worthless after 4 years whereas an Audi / BMW / VW etc will still have a good residual value.
To make it work the scheme would need better cars than most currently on it.
Maybe the 4 year lease would only be offered on certain cars.