Pokemon Diamond/Pearl Anti Elite 4Edit
Infernape/Rapidash (Infernape reccomended) for elite 4 no.1.
Abomasnow/Empoleon (Abomasnow reccomended) for elite 4 no.2.
Empoleon/Gyarados (Empoleon Reccomended) for elite 4 no.3.
Weavile for elite 4 no.4 but it's easier to use your team you have already.
Abomasnow is nice for Cynthia since he can take down Milotic, Roserade, Garchomp AND Gastrodon for his amazing type matchup so Abomasnow is a must. Spiritomb is not a big problem but it has hardly any weaknesses but he's not that hard to take down. As for Lucario use Infernape/Rapidash as fighting type beats ice type. So the reccomended team is:
Gyarados (since you started with Chimchar or if you can get an Empoleon by trade or Infernape by trade)
The last 2 pokemon don't matter but if you have Empoleon it would be nice to have a backup Gyarados and if you have Infernape it would be nice for a backup Rapidash anyway for the elite 4 all these pokemon should be at least level 50 and here are the movesets for the 6 pokemon:
Abomasnow: Wood Hammer, Ingrain, Blizzard and Ice Beam.
Infernape: Flamethrower, Close Combat, Brick Break and Any Move.
Gyarados: Ice Fang, Waterfall, Physical Water Move and Physical Water Move.
Weavile: Ice Shard, Night Slash, Faint Attack and Brick Break.
Empoleon: Hydro Pump, Surf, Flash Cannon and Drill Peck.
Rapidash: Sunny Day, Solar Beam, Flamethrower and Any Fire Type Move.
Those are the suggested pokemon want to comment? Do it below.
Feel free to comment. :)