20x4 LCD shows up as 20x2
Nori, just ignore every code that's after the //, they are just comments and the compiler will just discard them.

Look at the LCD rear, you should see your I2C chip, search google typing 
["chip's code"  20x4 arduino]. You'll see which I2C address you most likely need to use. For instance, the PCF8574 has addresses fromm 0x20 to 0x27, the PCF8574A has addresses from 0x38 to 0x3F...

Without this info you are only shooting blindfolded, the 0x27 it's only the "standard" address of the most common PCF8574 module but there are plenty of modules.

Another way to find the address is to upload this code, it's an I2C scanner that will write on the serial monitor any I2C device that replies a "ping" signal.

If you find the address and still it doesn't work or if you can't find the address, check and double and triple check the wiring, especially the false contact.