You can use Program 199 on the Korg Kaossilator Pro as a fifth track—and even merge it into the standard tracks.
The Korg Kaossilator Pro is a very cool 4-track loop recorder (see my “Quick Pick” review). But unlike the original Kaossilator, it can''t undo overdubs. And unlike the Korg KP3, it can''t bounce two tracks into one—unless you use this trick.
Record one to four loops and then switch to Program 199, Audio-Looper. This effect continuously samples the last four beats of audio arriving at the mic or line inputs. Press and hold the far right of the x-y pad, and you''ll hear any external audio you just played, looping. Messed up? Lift your finger, play again, and then touch the pad to capture your riff: It''s instant undo, without stopping the music. To merge your new loop into one of the other tracks, just hold that track''s button for the duration of the loop.