brute force 750 no 4x4
When you say the igniter is smoked, do you mean that just the 4WD function is not working or other function as well? If only the 4WD is not working, then the 12 VDC output from transistor circuit to the Control unit is smoked. And that's why 4wd is not engaging because Control is not getting 12 VDC from the igniter. If you have aftermarket Ignition Control Module (ICM) that spliced into the existing wiring harness, that's the cause. But, sometimes, that transistor in the igniter just smokes without any aftermarekt ICM. I suspect that's due to fluctuation in voltage signal from the speedo sensor located at the right side of quad, just below/behind foot brake pedal. That is the pink wire (speedo sensor voltage) connecting to the igniter. I would install an in-line voltage suppressor (also called noise filter) as a prevention - get one for $10 from Radio Shack.
Most quad problems are caused by the loose NUT connecting the handle bar and the seat.