Welcome to the forum.
It sounds like you've used a sensible approach.
It's a bit of a balancing act, but get all 4 plugs out and plug them into the coils and wires and crank the engine and confirm the all 4 plugs fire (two by two).
Preferably use the original NGK plugs and make sure they're gapped at 0.75mm (0.03").
It that all checks out OK, it may be a bad injector.
Quality and not quantity counts.
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