what is the thread size and pitch of the bolts which holds on the plate on the back of the stock s4 turbo?

pic included

 

Looks like 8mm x 1.25.

 

Same 8MM bolt that is used everywhere....M8 x 1.25.

 

If that is your turbo, do NOT use a plain steel washer for your wastegate flapper. Cut off an exhaust valve from a piston engine, it should be stainless and is made to hold up to the heat. I and others have had washers completely melt away, making for an exercise in futility.

 

That picture is actually from my website, and is two hardware store washers welded together. 35K later on the turbo and it's still fine.

But yeah, use a proper stainless washer, piston valve head, etc.