All charge values are a compromise between highest charge and battery longevity. There is no such thing as the 'right' value. There is a graph that can be drawn, with number of charges before the battery is junk vs final full-charge voltage. Maybe the line starts to rise/fall more at some point, so the mark goes there.
A compromise figure that leans toward a bigger charge (lasts longer before you need to charge it again) but with less battery life (max number of charge cycles) is 4.21 volts.
A compromise figure that leans toward max battery life but with a smaller charge is 4.17 volts.
You get these points off the graph. Most people prefer bigger charge and less battery life, which is why chargers cut off at 4.2 or 4.21 volts. To prioritise for overall battery life with one of these chargers, you'd pull the battery off charge before it gets to the top, and meter it (after a minute or two).
A charger that stops at 4 volts is probably faulty as AFAIK no one wants to promote battery service life to that extent - your battery will last a long time, but each charge is going to be on the low side as regards time to recharge.
Once again, there is no 'right' figure for any of these numbers, they are just pulled off the graph in order to give something to work with. But full charge at 4v is a bit too low.
[tech note]
No need to worry about battery memory any more, that doesn't apply to the Li-Mn cells we use now, it's a problem with ni-cads.
