You can map a button to the -2 and x2 buttons in the looper.
Once you've recorded a loop set at 2 bars for example, using these buttons would cut it in half to 1 bar or double it to 4.
When it doubles the loop, it keeps playing it so you can record a longer loop over the same repetition.
When it cuts it, the loop will only play half the length though-
Another method is to use multiple instances of looper. This way, you can keep all 4 original bars playing.
Also, you could just play the 1 bar 4 times over the 4 bar loop... not the most exciting thing though haha
Hope that helps!