Bear Spray is actually good for 8 years from the mfg date. The mfg tells people to replace it every 3-4 years because it could have sat on the shelf for 4-5 years. I always go by the mfg date. The bear spray never goes bad, but there is always a concern that the amount of pressure will diminish over time. I have a friend who works at the factory.Good bear spray has a shelf life of 3 years and costs less than $50 bucks, totally worth it for bears and also for people.
I almost always carry it in my pack. If I have concerns about bears I keep it on my belt. Works on many different animals that can pose a problem.