I2C Lcd 20x4 work exactly as 16x2.
You'll just have to modify the values in the library "LcdI2cLib.c" (I'll tell you how).
You must also determine the I2C address of your display (generally 0x20 or 0x27) and set it in the library "I2CLib.c"
But before, if you want I2C to work, you need to install Ldmicro32 v5.5 and the AvrGcc compiler from Github:
Here's attached a sample ld file for I2C 16x2 lcd.
I have tested it with my Atmega328 and it works.
Let me know if you need help.