Is it possible to install a service pack to a SQL 2012 Express instance?
I just now figured it out. right after posting the question... should have waited a few more min.

I was trying to install SQL 2012 SP4 but it wasn't letting me choose the express instance, to apply the update to, only our other standard edition instance. I realized I had downloaded the x64 SP4 package and that when the Express instance was installed it was installed with the x86 version. I downloaded the x86 SP4 package and now it is letting me update the instance. 

Maybe this will help someone in the future.