Hi Thomas: What type of external sounder do you have at the moment, is it one of the old wedge shape ?.
When was the Logic 4 last serviced ?. Does your sounder have a strobe fitted ?
To answer your question: For someone that knows alarms and is confident working at the top of a ladder, it should be an easy swap out. As long as the Installer used 6 or 8 core cable and it's not a panel fault causing your problem.