Good place to train after E4?

You could try the Big Stadium and Little Court, which has trainers with Pokémon around level 65.

Route 15 has some decent leveled trainers as well, and if you work your way toward Nuvema Town the trainers get progressively harder.

Otherwise, I haven't explored much. You can also battle Alder in Flocessy Town, who has a team around 60.

Edit: Seems you have to beat Alder to challenge Black Tower, just did it myself.

Black Tower's (and White TreeHollow's, I'm assuming) trainers have Pokémon starting around level 50 in Area 1, so it's not a bad training spot either. I'm in Area 4 and they're just now reaching 60.

I would have to go with Black Tower/Tree Hallow. There's, I think, 10 Arenas and the higher you get the person behind the gate eventually uses legendaries. So good exp is good. Plus more money
And a free shiny Pokemon

(I feel like I shouldn't post here.. All the mods.. ;.; )

Might want to replace Cobalion with some sort of a Flying-type, as Cobalion, Lucario, and Golem are all weak to Fighting-types. Golem can be replaced by some sort of Dark-type, so you actually have something against Psychic and Dark-types. I personally recommend Drapion (their pre-evolution, Skorupi, is available at Reversal Mountain), as it's fast, has a solid Attack stat, and can learn the "elemental Fangs" (Ice Fang, Thunder Fang, and Fire Fang; bolded Ice Fang because it's the more preferred of the Fangs). Other than that, I really can't see you not getting to Iris without much an issue.

Join Avenue cafes and dojos can level your Pokemon up easily.

Well, there are a few places that are good:

Hugh(Your Rival)
Old Team Plasma Base in Driftviel City
His Pokemon are about Level 62 and you can battle him once per day after beating him in Undella Town and receiving Dive

Alder
Floccesy Town
His Pokemon are high leveled, but he is only able to be defeated once.

Cynthia
Undella Town - Spring
She can be found in Caitlyn's house and has Pokemon level 76 - 78. She is only battle-able in the spring.

Benga(Alder's Grandson)
Black Tower/White Treehollow
Despite having only 3 Pokemon, all his Pokemon are level 80.

Game Freak
Castelia City
The 2 members of Game Freak have Level 76-78 Pokemon, being very difficult to beat. One has 4 Normal types and 2 Water types. The other has a mash-up of Dark, Water, Fire, Grass, Psychic/Flying, and Electric.