I changed all the plugs in the car today. Went with Autolite Double Platinums

I had a misfire on #4 that resulted in an SES light. And the other day when I got the wastegate solenoid replaced, the mechanic said he saw 2 more misfires in #2 and #3.

When I took the old plugs out they all had a white film on them. #4 was the worst.(whitest)

I'm wondering what might be causing this and what I need to look for/fix.