I found an article detailing how a normal Droid RAZR's battery was swapped and fitted with a RAZR MAXX battery after some small modifications.
Wish Your Droid Razr Had Maxx Battery Life? It Can! | TechCrunch
Battery Jealous DROID RAZR Owners, the RAZR MAXX Battery and Cover Will Fit Your Device – Time to Get Modding – Droid Life
This led me to search the Internet for a replacement battery for the Droid 4, and I found the following.
Motorola Droid 4 XT894 Battery SNN5905A
Now, after reading these articles and seeing pictures of the battery being removed from the Droid 4, I feel very optimistic about an aftermarket company being able to develop a larger battery and back cover for the Droid 4. If all that needs to be done is peel the sticker off and remove a few small screws then that seems easy enough. I have noticed that when I take the back cover off my Droid 4 there seems to be a little extra space in between that may be able to accommodate let say a 2,500 mAh battery. I would like to see templates for at least two different sizes, a 2,500 mAh and a 3,300 mAh battery that could rival the RAZR Maxx. Now, I know we are still early in the life of the Droid 4 and I don't have any intentions of trading my Droid 4 in for a RAZR Maxx but I still wouldn't mind having a little extra juice for my Droid 4. What do you guys think and would you like to see a larger capacity battery/back cover developed for our Motorola Droid 4?