1 - Events.
2 - Pal Park can be used to transfer from 3 to 4, but that needs a gamecube and GBA.
3 - You can get the earlier starters in Gen 4 via completing special storyline crap, check gamefaqs.
4 - HG/SS are in Gen 4, and cover gens 1+2...
And remember, starters are breedable, people often get one, breed it, and trade the babies away.