I have run into several cases where emax esc’s do not work properly with the pixhawk and even on the emax site, if you check the reviews on their esc’s there are many comments on them not working with the pixhawk. I beleive it is an issue with the firmware flashed on those esc’s that don’t sync up or simply do no interact with the pixhawk.
If you’re looking for an inexpensive 4 in 1 esc, I have personally tried the Quattro 4 in 1 25x4 esc and currently use a Q Brain 20x4 esc on a quad build using the pixhawk.