With Traktor Kontrol S4 MK3 it is missing manual loop in or out buttons.

I solved the problem by following.

X deck Set Loop IN: Shift + select deck X + reverse

X deck Set Loop OUT: Shift + select deck X + flux

X deck (move) mode selector: Shift + select deck X + push move button  <- use this to select Loop in / out

X deck move: Shift + select deck X + turn move button <- use this to actually adjust the loop length

X deck (move) size selector: Shift + select deck X + turn loop button  <- change how much you want to adjust the loop length

Sidenote 

I am a computer programmer (JVM languages) / aspiring DJ. It would be nice if in the future if I could contribute Traktor PRO software. :)