4 seconds for the R44 seems rather high now. I found this graph while looking around:
I think the text at the bottom goes something like...
t/k is the criteria used worldwide to qualify an helicopter autorotation ability. It is calculated by dividing the usable inertia to rotor stall by the hovering power. t/k give you an idea of the time to land? (in a hover) or time to enter autorotation (in cruise) after an engine failure at full-power. Above values are given at sea-level std day.