BLTOUCH
BLTouch: Is the solution to the problem that the PEI Spring Steel surface is not as level as the Anycubic Ultrabase build plate. Using it, with Marlin's firmware Mesh Leveling capabilities, 3D prints will adjust the nozzle Z level to match the build plate's level irregularity.
BIGTREETECH BLTouch V3.1 Auto Bed Leveling Sensor+1.5m Extension Cable from Amazon
Note: that the wire colors of the Extension cable are not the same as the BLTouch cable and needs to be kept track of when routing the cable to the controller board.
Note: the BLTouch kit doesn't come with an adaptor to mount to the 4MAX hot end.