I'm under the strong impression that people have different definitions of what "light" usage is, as well as incredibly different ideas on how long a battery SHOULD last.
Remember, the display is a HUGE drain on the battery. I know that on my OG Droid, the display alone will drain the battery in about 5 hours. That means playing games, going on Facebook, texting, etc. will drain the battery in about 3 hours once you add in the network and CPU usage. While the Droid 4/RAZR have a larger battery than the OG Droid, they also have more powerful CPUs, more RAM, and faster network capabilities. In the end, it probably breaks even.
I'm getting about the same battery life with my Droid 4 as I did with my OG. I will read Rage Reader during the ~hour and fifteen minute ride to class, then hop on Facebook/text people between (and sometimes during) class, and at my battery is at 20% or less when I leave my last class. I don't consider this to be poor battery life. It adds up to at least 2 hours of screen time, plus CPU/network battery usage.