Well, the code sample you posted initializes a 16x2 LCD on an address of 0x20. You're trying to initialize a 20x4 LCD on an address of 0x27. This might call for a change to the sketch, wouldn't you think?
It's also possible that you have the wrong LiquidCrystal_I2C library installed--there are at least two different libraries with that name. I have an LCD that uses one, and the library I downloaded here under that name didn't work. Make sure you're using the right one.