Some make the assumption that there is a ballast resistor inside the coil but I don't think that is quite accurate.
Well, there has to be extra resistance somewhere, to limit the DC current through the coil once the magnetic field is saturated. Normally, the length of wire used for the primary isn't long enough to do that; so either there is a discrete resistor inside, or the wire is a special alloy with higher resistance than copper.
Some refer to the 2 styles as 6V and 12V coils which is accurate but possibly confusing. In that parlance the TR4 uses a 12V coil.
But I have coils that are marked "For 12v use. Requires external ballast".
To add to the confusion, there are various coils designed to be used with various electronic ignitions, that have even lower primary resistances. Those ignitions have their own means of limiting saturation current (eg adaptive dwell or capacitive discharge). So, for example, Pertronix offers canister type coils with primary resistances as low as 0.32 ohms!
Likewise, different electronic ignitions have different requirements. So for example, a Crane XR700 needs a 3 ohm coil (or a 1.5 ohm coil plus a 1.5 ohm ballast), but a Crane XR3000 takes a 1.5 ohm coil. And Pertronix actually specifies lower resistance coils for V8 applications, for example when using an Ignitor ignition on a 12 volt V8, they say to use a 1.5 ohm coil instead of the 3.0 ohm used on 4 & 6 cylinder. (6 volt V8 applications use the 0.6 ohm coil.)
So IMO, the safe way is to talk about primary resistance. Stock TR4 coil is a 3 ohm coil.