[REQ] the last but four line?

c.

A and B are not valid. C is good.

[E] the fourth to last line

E is also good.

But I sometimes hear such usages as "the last but one line" or "the last but two line" or "the last but second line".

Well now you can correct them.

I think all your alternatives are "correct", but counting down from the top of the page is the most familiar method of noting a specific line.

You do???

"the last but two line"

You know that "two" is a noun...

Stuh--

"Last but two" is common in the UK.

And in the US people say

"how you doing?"
"I'm doing good."

but that "don't make it right".

Ah, Stuh. You're a verbal, international idealist.

Nah...I understand that language is just a tool for communication, and that it's usage eventually redefines it's proper definition. If people want to misuse the language, that is their perogative. Heck, I do it...but that does not mean we should say "to hell with the rules" when we teach it / learn it.

Maybe some people in the UK talk like that but if they do, it's colloquial slang, and certainly can't talk or write like that in the US without people thinking "you don't know English!" And I think, in general, people on this forum want to learn how to speak American English.

Yes and know.

We have some British members, some Australian members and a few South African members--to say nothing of our third world posters from former British colonies.

Oh, Brave New World!

Sounds right, but is it?

Stuh--

So much in this world depends on context.