Help me 4 Banks usbMIDI arduino MEGA

Hello help me with something I can not understand, how can I add 4 banks to this code, I use 16 digital pins as buttons and I would like to use 4 additional buttons to select the banks, I would like to add this to the code I use But I do not know how to do it

//BUTTONS x16
int buttons[16]= {
1, 5, 10, 14,
2, 7, 11, 15,
3, 8, 12, 16,
4, 9, 13, 17
};

// FORMAT <----> //"BTN#" "NOTE"\
//----------------------------------------------------------
// BANK 1 BANK 2 BANK 3 BANK 4
// C-1 C#-1 D-1 D#-1
//----------------------------------------------------------
// 1 - C3 | 1 - E4 | 1 - G#5 | 1 - C7
// 2 - G#2 | 2 - C4 | 2 - E5 | 2 - G#6
// 3 - E2 | 3 - G#3 | 3 - C5 | 3 - E6
// 4 - C2 | 4 - E3 | 4 - G#4 | 4 - C6
// 5 - C#3 | 5 - F4 | 5 - A5 | 5 - C#7
// 6 - A2 | 6 - C#4 | 6 - F5 | 6 - A6
// 7 - F2 | 7 - A3 | 7 - C#5 | 7 - F6
// 8 - C#2 | 8 - F3 | 8 - A4 | 8 - C#6
// 9 - D3 | 9 - F#4 | 9 - A#5 | 9 - D7
//10 - A#2 | 10 - D4 | 10 - F#5 | 10 - A#6
//11 - F#2 | 11 - A#3 | 11 - D5 | 11 - F#6
//12 - D2 | 12 - F#3 | 12 - A#4 | 12 - D6
//13 - D#3 | 13 - G4 | 13 - B5 | 13 - D#7
//14 - B2 | 14 - D#4 | 14 - G5 | 14 - B6
//15 - G2 | 15 - B3 | 15 - D#5 | 15 - G6
//16 - D#2 | 16 - G3 | 16 - B4 | 16 - D#6

//TOPBANKS x4
int topbanks[4]= {
18, 19, 20, 21
};

The code is on this page http://www.instructables.com/id/Arcade-Button-MIDI-Controller/step7/Programming/
The name of the file where the code is located is called MIDI_Controller_2012-01-20.zip