Lock Nut Replacements

[jkonline]

Somehow the lock nut on the front wheel of my b4 fell off after I crashed.
As I can not longer find it I need to replace it as a three wheeled car is just not the same.

I could replace it with the official part 6222 labelled "4-40 or 5-40 Black nylon self threading locknuts" however I was wondering was it worth changing to aluminium e.g. something like losi 4-40 aluminium low profile locknuts? is there any point and will they fit.

If someone could also explain what it mean by 4-40 or 5-40 and which I will need?

Thanks