Button Sequence Input

[cgmv123]

int state = 0;

while{
if buttonA && state == 0
state ++;

if buttonB && state == 1
state ++;

if buttonX && state == 2
state++;

if buttonY && state == 3
state ++;

if buttonBack
state = 0;

if state == 4
{action
state = 0;
}

This is Java-focused pseudocode, but it should be enough to get you started.