First get the servos working. Use one of the servo examples such as Sweep. Just do one at a time. When they are all proven to work, put them all together - this tests the power supply can run all 4 at the same time.
Then read Serial Input Basics You can't just wait until there's 2 characters in the input buffer.
I can't help you out with programming the app on Android. I hope that you have some resources to do this. I believe the MIT App Inventor kit is pretty good.