GFK's #4 sounds good to me. If I could figure out how to rig it up so it would drop firecrackers and light them before dropping them, I would take care of the neighbor's cat who keeps crapping in my wife's flower beds.
Going off-topic a tad, but I got some of this from eBay, although you can get similar things in garden centers too.It stinks rotten (to cats, and also works on dogs). Just scatter it around on the flower beds and cats won't go anywhere near it. It does discolour the leaves on certain plants though, so make sure not to get it on them if you can avoid it.
Actually it doesn't smell that nice to me, either. But the smell goes away after a couple hours (but it continues to keep cats away for weeks and eventually they'll just crap some place else)