I'ma write one for Ninja Frog.
Greninja @ Mystic Water/ Black Glasses
-Extrasensory/ Ice Beam
-Dark Pulse/ Night Slash
Why is his tongue a scarf? The foam he had as a Froakie/ Frogadier would be so much cooler.
Dark Pulse or Night Slash, whichever's easier for you. Greninja can be a good mixed Pokemon in-game. Water Shuriken is actually more useful than you'd think, because of those Gravelers and Boldres; Water Shurkien will break Sturdy. Either Extrasensory for Fighting coverage or Ice Beam for Grass coverage, so it would depend on the rest of the team. Or you can sacrifice Water Shuriken for one or the other. Considering that I'm advising Venusaur, I think Extrasensory is more beneficial, because Venusaur can handle grass types.
Acrobatics is another moveset option, if you so felt like it. It'll most likely OHKO Chestnaught if you're a few levels above it.
Greninja is a really solid starter; great speed, decent attacks and some nice STAB moves.
Weak to: Electric, Grass, Bug, Fighting, Fairy
Resists: Water, Fire, Steel, Ice, Dark, Ghost
Immune to: Psychic
Venusaur @ Miracle Seed/ Poison Barb
-Venoshock/ Sludge Bomb (you get Sludge Bomb really late)
-Earthquake/ Giga Drain/ Petal Dance
I was going to go with Charizard at first for the Fairy resistance, but Venusaur actually has super effective moves against Fairy types (that don't have a 75% accuracy like Iron Tail), plus the resistance. Sorry Mega Charizard X, as cool as you are, Venusaur wins. Earthquake for coverage and to deal with stuff like Klefki, which has has a Poison immunity because of Steel typing, or another STAB Grass move if you wanted either a draining move or a more powerful grass move.
Weak to: Fire, Flying, Psychic, Ice
Resists: Grass, Fairy, Electric, Water, Fighting
Mienshao @ Expert Belt/ Black Belt
-Power Up Punch/ Rock Smash
I'm so glad they included Mienshao in this game; while there are plenty of fighting types to choose from, not least that weird new Fighting/Flying Hawlucha, but for me Mienshao wins out because of Regenerator. Rock Slide will be sufficient for in-game purposes, and Mienshao gives a nice Rock resistance as well. I'm not going with High-Jump Kick, since if it misses you're really screwed. You should really only use Mienshao against things that are weak to fighting anyway, so you might as well go for Power Up Punch & then Drain Punch on everything.
Rock Smash is actually pretty useful, since there are plenty of rocks in your way in this game, and because it's not a HM move, you can replace Power Up Punch with it and swap the two moves contiually.
Mienshao does evolve late though, and you might think that Regenerator doesn't matter too much because you can use items, but +30% makes quite a big difference. Mienshao is worth using if you're patient.
Weak to: Flying, Psychic, Fairy
Resists: Rock, Bug, Dark
Mewostic @ Mind Plate/ Leftovers
Ability: Keen Eye/ Infiltrator
-Reflect/ Light Screen/ Charge Beam
-Signal Beam/ Energy Ball
Competitively, Prankster (male) Meowstic will be such a good support Pokemon, since it can set up at least one screen because of the ability.
In-game wise, either Infiltrator or Keen Eye is fine. There are a few select trainers that do set up screens, but beyond that Infiltrator is useless. Keen Eye is probably the better ability taking into account that there are SO MANY Boldres in this game and they usually have Mud Slap. Mewostic also learns some good special moves beyond its Psychic STAB, and can have some really nice coverage.
I would personally advise the male Meowstic, simply because I like having a defensive Pokemon around, but the female Meowstic is one with better offences, and in-game battles are hyper offensive, so you might want a female one. If you do get a Female Meowstic, Charge Beam is an option for her over a screen, if you want to use her to sweep.
Weak to: Bug, Dark, Ghost
Resists: Psychic, Fighting
Sylveon @ Pixie Plate
Ability: Cute Charm
The star of X&Y, Sylveon is actually a pretty decent Pokemon. It's surprisingly slow, but it has some really great HP & Sp. Def, along with a nice Sp. Atk. Draining Kiss to replenish health, and it learns it very early on at level 20, so either evolve Eevee at level 19, or use a Heart Scale. Moonblast is great STAB, at BP 95, 100% accuracy and a chance of lowering an opponent's sp. atk. Psyshock and Dig to cover its weaknesses, even though they're pretty uncommon in-game, but Psyshock will prove really useful against Toxicroak (which, spoilers, Team Flare uses). Sylveon will trash any Dragon types, with the possible exception of Dragalge. Psyshock is only available later in the game as a TM, so Bite will be a good filler move until then.
Weak to: Steel, Poison
Resists: Fighting, Bug, Dark
Immune to: Dragon
Emolga @ no item
-Volt Switch/ Encore/ Light Screen
You can get Emolga on the same route as Eevee, at a relatively infrequent encounter rate, and as far as I can tell, Emolga has the upperhand in sky battles. It also learns Acrobatics, which will be great against the Fighting Gym, and against some dude's Jumpluff in a sky battle which I was not expecting. It's got pretty cool typing, meaning its Ground weakness is negated, and its Rock/ Ice weakness is covered by Mienshao and Greninja respectively.
Sky battlers also use Emolgas themselves, but just use a STAB Electric move and you'll be fine. You should usually be a few levels above your surrounding trainers/ Pokemon anyway.
Emolga is boss.
Weak to: Rock, Ice
Resists: Bug, Grass, Fighting, Flying, Steel
Immune to: Ground
I didn't include most HM moves, but guess what? Bidoof is BACK. So go use a Bibarel. As for Fly, I personally didn't find it that much of a deal to forfeit Fly, considering that you can just Fly to a centre & swap Pokemon. You can use a Fletchling to Fly. The tiny bird doesn't even need to evolve in order to learn HM02.
Also note that these are final movesets, so mess around with the moves, use HM moves or whatever throughout the game, but these are the sets I recommend for E4 purposes.