I'm moving out of my current place in a few days. When I'm doing something a little annoying to my roommate, I joke, "sorry, this is (will be) like the 4th last time I do this", to mean that I'm only gonna do it 3 (a few) more times, so don't sweat, and forgive me. Am I using "4th last time" right?
I would always say 4th to last time.
"Fourth last time" sounds like it's the 4th time you're saying it for the last time.
Yes, if after saying this, you will, or are planning to, do the same thing 3 more times, then you can definitely say, "Sorry, this was like the 4th last time I do this", or more like "4th last time I plan to do this."
I see nothing wrong with the use of "4th last time" here.
I read the statement
"this is like the 4th last time I do this." to mean you have stated
"this is the last time I do this" three times already.
Unless you have said this three times, then I would say you are not using the word
"4th" correctly in this manner.
If you are trying to say
"I am only doing this three more times", then you could say the phrase
"sorry, I have to do this three more times."