All five regions of the main series games have their own Elite 4, including Johto, but it's more of a transaction of three years from the events in Kanto, with both sharing the name of the Indigo League. Each Elite 4 member specializes in a specific type, though in some cases the trainer may not have every pokemon with a mutual type(like the generation one Kanto Elite 4, due to pokemon amounts).
Here's the Elite from each region(In order):
Kanto:
Lorelei-Ice type user
Bruno-Fighting type user
Agatha-Ghost type user
Lance-Dragon type user
Johto:
Will-Psychic type user
Koga-Poison type user
Bruno-Fighting type user
Karen-Dark type user
Hoenn:
Sidney-Dark type user
Phoebe-Ghost type user
Glacia-Ice type user
Drake-Dragon type user
Sinnoh:
Aaron-Bug type user
Brenda-Ground type user
Flint-Fire type user
Lucian-Psychic type user
Unova(from left to right):
Shauntal-Ghost type user
Grimsley-Dark type user
Caitlin-Psychic type user
Marshal-Fighting type user
(generation 1-4 Elite 4)
Now, for the trivia!
Unova's Elite 4 is the first one to allow any order of the Elite 4 to be battled, thus having each member having their pokemon at the same levels.
It use to be that the Elite 4's pokemon levels were set no matter how many times you battled them. In Firered and Leafgreen, after you accomplish enough in the post game, you can re-battle the Elite 4 with higher leveled pokemon. This came back in Platinum, and has stayed continuous with every main series game since.
The standard for an Elite 4 member is to have five pokemon each. Unova broke this once again having four for the first battle. More so, when re-challenging the Elite 4 in Heartgold and Soulsilver after accomplishing enough in post game, each Elite 4 member have six pokemon for all battles. This has continued on in Black, White, Black 2, and White 2 in post game battles.
Will from the Johto Elite 4 has the lowest level pokemon with his Xatu at level 40 in all battles in generation two, and before the level increase in Heartgold and Soulsilver. All Unova members have their respective highest level pokemon as the highest of all Elite 4 members with level 79.
As you can tell from the types of the Elite 4, the Ghost, Dark, Psychic, and Fighting types all have been in the Elite 4 the most times: Three times each.
All of the members from the Kanto Elite 4 and the Sinnoh Elite 4 have made appearances in the anime.
It is unlikely to meet a member of the Elite 4 outside of the respective League you battle them in.
In Black 2 and White 2, there are an assortment of medals to obtain by beating the Elite 4 with pokemon sharing a common type, as well as beating the Elite 4 with just one pokemon!
And a question to leave: Which is your favorite Elite 4 member?
Unova Elite 4.
