You could still run shouler bolts, but you need to heat up the grub screws on the hanger, take the grub screw out first.
For the difference: Gen3 is 340mm hanger wide, axle spacer is 22mm wide, shoulder bolt axle. 960g per truck.
Gen4- hanger 330mm wide, axle spacer 16mm wide, using real axle with nuts, also added grub screw on secure the kinpin.
800g per truck.