When the main needle is too rich,it may happen. Think about the .12 engine is so small size,if you drop 2 drops fuel in the combustion chamber,it is hard to turn the piston to the TDC.

Another reason maybe the starter box is too weak. G4 has the smallest flywheel,so the start torque is small too. Recharge the battery of your starter box.

I don't think the plug reference to this problem. But the weather is hot now,a #5 plug may be too hot.