I have a log that looks like this:
How can I use AHK to delete lines 1, 2, and 3, but skip the fourth, and then continue to loop?
Your using the wrong tool for the job. (Not to mention that how to use software is a superuser.com question)
The solution is to open up a terminal and type:
tail -n+4 yourFileWithLines
That will work perfectly. On windows install cygwin to get tail. (<== That could almost be a twss joke)
This is not the direct answer you are expecting, but if you install cygwin (I guess you are working in Windows) and try grep & less. You will soon realize they are the right tool for your purpose.