How the (4 column x 4 row) keypad works:
When you press a key, it brings one of 4 pins LOW (depending on which of the four ROWS the key is in), but only if that particular column is selected by one of four ACTIVE LOW output bits .
The four pins indicating which row/rows is/are activated by a keypress are connected to the LOWER nibble of an 8-bit digital I/O expander for the I2C bus (address 33 in decimal).
The four pins selecting which column(s) are selected are connected to the HIGHER nibble of the same expander port.
To disambiguate key presses, you need to scan through the four columns of the keypad.