Put a cam in my brothers car. Well we went to tune it after tune it ran great for about a minute then it started misfiring. Back 2 cylinders dead on each side. Well tuners thought it was an electrical problem of some sort but we couldn't figure anything out. Well today my brother pulled the valve cover of the driver side and found broken valve seat pieces. What are the odds of some of these pieces making it into the block? Also can this cause 4 dead cylinders?

 

No way no how. The valve seats are what the valve seals against inside the cylinder.
Is he talking about spring seats?