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Hi. Have they checked the headgaskets yet?
No it can leak into the cylinder. And those heads are actually common to crack between the valves.
But yes over time the coolant level would go down. But it does not take much of a leak to cause problems in a cylinder.
Sometimes on first start up. It can cause the cylinder to lean out and burn your plug. If the shop had it for 6 weeks I am assuming some basic stuff must have been checked. A leaking head gasket can also cause overheating when putting a load on the engine.
How would cylinders 3 and 4 have bad plugs when in your question you stated that you just replaced them?