l usually take Claritin, but l ran out and my mom gave me an old prescription of Clarinex (like 4 years old!), and l dont think it is working....Should l just go buy more unexpired Claritin?

You should never take a medication that has expired more than one year ago. Expiration dates r based on drug stability & toxicity. Some drugs simply breakdown & lose their active ingredient as time goes . Other medications slowly change into toxic breakdown forms when not held in the proper storage conditions or after the expiration date. These r all things carefully validated with the manufacturing process & approved by the FDA.

You should go out & buy more Claritin. It is very cheap now. Some stores sell generic bulk pills (50) for $15.

Claritin/Clarinex do not go toxic after its expiration date.
The expiration dates are very conservative. Drug companies have to test the drugs for stability. They can theoretically determine the day which the drug dips below the 90% effectiveness. However, they generally determine the expiration date to be the last day of their testing.
Generic Claritin (loratadine) is fairly inexpensive.