Start from the outside in (what I would do)
: Remove the first spark plug with the spark plug wire and, with the ignition on (by cranking the engine), ground the side of the spark plug. Check for a spark, and do this with all four. If some spark plugs produce a very small spark and others a slightly stronger one, the problem is with the wiring, which may be retaining some moisture
. If you have a multimeter, measure the current at the coil input. If it's 12 volts or slightly more (to do this check, you have to try to start the engine and measure the voltage), the problem isn't with any relay, but with the engine itself. Check that the wiring is correctly connected. I'll keep you updated.
Check the spark plugs when the engine isn't starting properly