TextWrangler grep {4} basic problem

I learn things by taking a problem I have and using a new thing as practice.

GREP is the current thing to learn. I have a .txt file with a list of Apple file paths so there are a lot of colons. Specifically, I want to copy everything after the fourth colon to the end of the sentence. Then eventually paste that and paste it again after the find putting a tab character in between. I'm stuck on finding.

Setting the 'grep' option in TW's find/change dialog, I try out this:

:{4}$

Beeps, so I try it without the end of line character. Beeps again, so I change the numbers, still beeps. Only when I leave the colon by itself does it find all the lines with a colon. That doesn't help me. So I'm doing something wrong with the number character. What am I overlooking? thanx, sam