1 in 4000 over a lifetime is not a fairly large reduction of risk - it's a very small reduction of risk. There's a much bigger payoff in thinking about the 3999/4000 chance that something else will kill you.
I feel those are bad odds. Plenty of other people are happy with them.
Also, that's 1 in 4000 before 60 (60 - 10 = 50). You have a 12% risk of death before 60 which means if you died before 60 the there is a 1 in 500 shot that it was biking without a helmet that killed you. Granted a significantly increased change that it was biking, but using a helmet would not have saved you.
I would agree that biking after 60 without a helmet does not really increase your death chances that much.
