Four units of alcohol a day over what period of time?
One year? Two years? Ten years? 30 Years? 50 years?
It is an extremely important number.
Apparently it’s more the number of drinks added in a day that is exponentially damaging rather than the length of time per se. So when you go from abstention to half a beer it isn’t so bad but from half to one, 2 years damage. Two to three, 3.5 years etc. In other words there may be safe amounts but basically the less the better.