I am going to take a stab that you changed your resolution. For some reason the market won't let you install if your resolution is not default. I guess it is just meant to be a sanity check.
If this is the case, you will need to reset your resolution (I recommend ROM Toolbox) and then go into settings and wipe your market (play store) cash and reboot. I think the default is 240, but that could differ from device to device. I haven't checked what it is for droid 4. It was 240 for droid 2.
Pain in the dairy air because I do like custom resolutions. It just got so tiresome I finally just left it default. Would be cool to find a way to fool it some how.
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