Prince Durin IV

I don't think it at all uncommon to number your children when they have the same name as you, royalty or not.

John Doe has a kid, names him John Doe II, who has a kid named John Doe III. Helps tell them apart. ?

As Amazon has decided to have Durin III and Durin IV alive at the same time (due to their time compression), it makes sense to refer to them by number all the time.

But it is different for royalty.

Lets say King John Doe I has a child called Prince John Doe II, who then in time on the death of his father becomes King John Doe II.

King John Doe II has two sons also called John, Prince John Doe III and Prince John Doe IV, however Prince John Doe III dies before his father, so Prince John Doe IV succeeds as King John Doe III.

This is why princes don't have numbers.

Doesn't make sense (I mean, I follow you, but having separately numbered princes just baffles me personally ?)

Still, Amazon is using Tolkien's numbering from the Appendices - so they aren't following "normal" rules, and the time compression has broken what Tolkien intended (that no two Durins would be alive at the same time).

We can all go around referring to him as "Prince Durin who will eventually be IV", but I don't think Amazon will follow along. ?

I am hoping that in the show he is referred to as Prince Durin, hopefully Elrond does not announce that he is going to visit Prince Durin the forth.