Before one is carried away with the thought you may have something very rare it is important to ask yourself - If others say something is R4 or R5 what are the chances of that being true ? So let’s think about that. An R4 rarity is 8 to 16 known examples in VF condition. What does that translate to in terms of percentage of what was originally minted ? A look at that the table opposite throws some light on this point.
Here the best estimate of number of coins in existence for each denomination is shown. Percentages for each rarity / denomination are shown below. Personally I feel confident down to a level of around 1% for an R4 crown. This is at the edge of the green area. Past that point in the yellow area it is more a guess unless at sometime someone has done a physical count of what’s out there, discounting contemporary fakes. Other denominations have a different threshold for that 1% level, so it’s well worth keeping this in mind.