G is right . 0.3 ohm is standard and about what mine measure out at . If yours are both 4ohm they may not be the original ones . Different ignition systems will have different dwell periods. The dwell angle is the time expressed in crankshaft degree's that the primary winding on the coil is energised. Im picking that the digiplex has a very short dwell period which means that the low resistance will allow a lot of current to flow in that short time to properly charge the primary winding . A higher resistance primary is better suited to something with a longer dwell.
My Guzzi came with stock coils ( 3.5 ohm ) and a fried Dyna cdi . I checked the spec's which said the dyna would work with stock 3.5 ohm coils so I bought a new Dyna and fitted it up , the coils were smoking hot within a few minutes and so was the new dyna ( hence the fried dyna it came with ) so I logic-ed it all out and fitted a 1.5 ohm ballast resistor to each of the coils ( in series ) end of problem..............'cept being a bit OCD I couldn't leave it at that and fitted some dyna 5 ohm coils. The point is that the coils do need to be matched to the cdi, to much resistance will probly leave you with a weak spark , not enough and stuff starts getting hot . Its not just the primary resistance that effects the coil output. The primary resistance controls how much current flows ..... the dwell time controls how long that current flows for.
Having said all that it beats the shit outa me how a iggy system designed to run a 0.3 ohm coil will work with a 5 ohm coil ....... but it does ..... must be something sneaky goin on in that digiplex
