Hi!
Sorry! But it's a simple fact that when an engine blows plugs it's too lean!
You cannot trust a tachometer reading only...it says nothing! You have too listen to how the engine sounds and draw your conqlusions from that. OS 8 or Enya 3 is good plugs that each one should last for years...if engine is set correctly.
And 11x4 is awfully small prop if you fly at sea level. A much better size for general sport flying is a 11x6 (APC) or 12x4 if you fly a high winged trainer or slow flying model with lots of wing area.


Regards!
Jan K