Fazer gen1 fzs600 1998, jitter between 4-6000 rpms (after fixing one plug)

[unfazed]

Change the 4 plugs then you can rule out a plug issue.

Unscrew the plug caps and cut 10mm off the ends and screw the caps back until tight.

Balance the carbs, set the idle speed and finally set the TPS.

Let us know how you get on