god mode....... Z-E-U-S (with god mode on, 'G' will toggle gravity)
warp mode...... N-E-X-T
increase ammo.. X-T-R-A

NOTES: For 'warp mode' and 'increase ammo' you must release the third key
of the four-key combination before you release the other keys.

EXAMPLE (warp mode):
First, press and hold: N E X T
Next, release: X
Then, release the other three keys.