By XML error, do you mean an error on loading or an error after making the changes?
I assume you know that all Col files are Read-Only, meaning you must change this before you are allowed to make changes. For instance, in Notepad++, if you don't do this, the editor doesn't recognize the fact that you did any change to the file.
By simply changing the value of <iMaxWorker> from 3 to 4 (after making the file non-read-only) in CIV4BuildingInfos.xml, I was able to load the game fine, start a game fine, worldbuilder myself a city, and worldbuilder myself a Coat Factory. Going into the city screen revealed I could indeed have four workers there.