i work for a Mopar dealer and i know our Sentry Keys 'can' lose their programming from time to time. doesn't happen very often, but it HAS happened. this is what i think happened to YOUR key, especially since you said your spare key works, but not the one you've been using.
the 'lock and unlock' function is the only thing that works off of the battery. the key itself has a chip in it that communicates with the immobilizer unit. for some reason, the immobilizer is no longer recognizing that particular key.
like "spikey01" said, you may have to contact someone that handles parts and service for the 450's and ForFour's. you may have to purchase a new key and have it programmed to your car, which sounds VERY expensive to me.....GOOD LUCK!
