What you could do is create a stack of, say, four variables. You'll need a fifth variable to keep track of the number of key presses. You'll also need a timer.
When you register the first key, you'll place its value in the first variable, increment the fifth variable, and start the timer. If you register another key press before the timer is finished, you place its value in the second variable, increment the fifth variable, and restart the timer.
When the timer ends, or all four stack variables are filled, then you can execute your "combo move" if it is valid combination.
Hope this helps.