In your TODO.txt you mention 'line numbers' and it was one of the first things I checked for when I booted up 4ed for the first time. Vim is my current editor, and one of its most valuable features for me is line numbers and relative line numbers. Even though this is something already on your TODO list, I thought I'd request something a bit more specific.

A gutter along the side of the text which is modifiable. Colors, buffer, size, etc. This would be very useful for things such as line numbers (relative or absolute), colors for denoting when a line has been modified and not saved yet, an editable buffer for marking a line of code as you please, etc. This would be extremely useful for the way I'll be using 4coder (and other people as well, I hope).

If you have any tips and tricks for how to accomplish some of this in the meantime, that would be great. I've been trying to mark lines in other ways and haven't come up with a good solution yet.