I was out for a charity ride a few weeks ago and as I was passing by people who were cheering us on, I used my horn lots of time to acknowledge them... the more I rode on, the more I realized that the little "meep meep" horn that is on the NM4 is both out of character and a safety hazard (little attention from cages).
I did a little research and decided to replace the horns on both my NM4s with the FIAMM Model F72112 "Freeway Blaster", which goes for around USD$16.. note that I went with the "Low Note" version (there is a "High Note" version as well, but the Low one seems to get the most attention judging from online reviews).
Installation was a breeze -- about 15 minutes to remove the old horn and replace it with the new. You can re-use the 10mm nut from the old horn and connection is via two spade connectors (polarity dies not matter according to my research).
The difference is quite discernible, with 130db when you hit the horn button!