Site I used ( today) clearly says BOTH the B8 and B7 are replacements for N4. How can that be?
Confused in NC…….I knew better.
I have a bsa article somewhere which kinda agrees with your B8ES =N4, though modern cross references say different. In this particular article it goes on to show the N3 equivalent is equal to B9ES.
Apparently NGK and Champion spark plug numbers are about half a heat range offset from each other+ as well as running in opposite direction, so the Champion N4 is about an NGK B7.5E* and the N3 is B8.5E*. Alternatively, an NGK B8E* is about a Champion N3.5 and the B7E* is about a Champion N4.5.
Just in case we weren't confused enough already, Champion's 'N' is 14mm, 3/4" reach
(yes, they really do mix metric and imperial), NGK's 'B' prefix is 14mm and 'E' suffix is 3/4" reach
[+] I read that years ago, but don't remember where, or even if the source was trustworthy. Take it with the appropriate sized grain of salt.