The JM4 does not offer the reverse loop feature.

It also does not offer an auto-record feature. (any "advanced" looper should have this feature for sure!)

It also lacks a redo feature (as in undo/then redo - again, any "advanced" looper should have this feature also.)

Also, it has a minimum loop time of about 2 seconds, which makes it very uninspiring for looping quick tap/slap backing rhythms with your acoustic guitar.

Also, if your primary instrument is acoustic guitar, you are limited to using the aux input mixed with vocals (to get a natural acoustic sound) which means you can't use any of the amp models, cool effects, or use any of the center control knobs on the unit as they are all designed to work with the electric guitar channel only.

That said, the New M9 Stompbox modeler has a looper with the reverse and undo/redo features.

It seems highly illogical for the JM4 which is marketed as a dedicated looper to lack these valuable features and then for the M9 which is marketed as a stompbox modeler to to include these features that the JM4 lacks.

Don't get me wrong-the JM4 is cool-the live backing tracks and effects are great, and the SD card functionality is convenient, but if you want a looper that is designed for live solo performance (which usually means acoustic guitar)  there are others loopers out there that are better equipped for the gig.

I was super stoked when I bought mine, I use many other line 6 products and I assumed this would be the best looper available.  Once I started exploring the features and discovering the limitations, I started encountering one disappointment after another-primarily those I mentioned above.

Hope this was helpful.

Good Luck!