Well, to defeat the E4 try:
Hariyama: It can have the ability Thick Fat, which helps against the Ice E4. It has Fighting STAB, which deals with the Dark and Ice E4, plus works against most of Steven's Pokemon.
Aggron: Steel is a type with many resistances, and will be useful here.
Walrein: Another Thick Fat Pokemon. This one hs the useful Ice STAB, to take down the Dragon type E4.
The starter: Of course you should have the starter on your team.
Kyogre: Helpful against the E4.
Camerupt: Mainly here for being a Fire type, although it also has STAB Earthquake. This guy is useful in taking down Stephen.
If you don't want to use Legendaries, then Manectric is a good choice, helping here and there.