Codes P0302, P0304, P0300, and P1399 (misfires) on a 2000 Honda Accord 4 cylinders?

I checked the spark plugs and cylinders 2 and 4 seems to be not tight enough. This may be the cause. But does a blown head gasket also causes these errors? I read somewhere that codes P030x on adjacent cylinders usually indicator a blown head gasket. In this case the two cylinders are not adjacent. I cleared the codes after tightening the two spark plugs and drove the car around the block and no error code so far. But this worries me a bit if the problem is more serious.

You are thinking way ahead. Get a spark tester and check for spark, get a noid light and check if the injectors are getting command to fire, get a fuel pressure tester and check fuel pressure. Better yet, get a tune up first. Maybe that is all it needs.

Low compression in adjacent cylinders indicates a blown gasket between those cylinders. That does not keep plugs from sparking. Check the resistance of the spark plug wires if the misfire persists after changing the spark plugs. The distributor could also have problems- check the cap for tracings and cracks.