How do I catch Charizard, Bulbasaur, Squirtle and Pikachu on Pokemon Platinum?

The question is pretty self-explaining. Is there a way to catch these Pokemon on Platinum? On a low level preferably in order to train them with good stats.

Charmander, Bulbasaur, and Squirtle cannot be caught in Pokémon Platinum, likewise for all of their evolution chain. They can only be obtained by trading from a different game. You can either get them through FireRed/LeafGreen via Pal Park (a place to trade Pokémon from the 3rd generation), or you can obtain them in HeartGold/SoulSilver and trade them over.

Pikachu can be found at Trophy Garden, which is the grass behind the Pokémon Mansion south of Hearthome. They should be available at all times, if my memory serves correctly. I believe they're no higher than 20, which is plenty sufficient to get all the EVs you need.

A note, however, is that you don't need to worry a lot about levels on non-legendary Pokémon that you catch. Any breed-capable Pokémon can be hatched from an egg, which results in a new one starting at level 1. This is also advantageous as you can use breeding to get a better moveset than naturally possible. So even if circumstances result that you can only trade a fully levelled one of any of these 4 Pokémon, you can always breed it to get a low level one ripe for your own training needs.

The three starters aren't available in Platinum, although you can get one in HeartGold/SoulSilver from Professor Oak after defeating Red.

Pikachu can be found in the Trophy Garden behind Mr. Backlot's mansion.

You can get the three starters in Platinum using Wi-Fi trade, providing you've seen them in-game. I don't know if any trainer in Platinum has Bulbasaur, Charmander or Squirtle. There are trainers with these Pokemon in HeartGold and SoulSilver.