How to ev train a Pokemon?

Okay so i have a marshtomp and i'm trying to boost its speed. I went to sprout tower in pokemon soul silver and I knocked out 20 rattatas. Plus, I infected the marshtomp with pokerus. The 20 rattatas should have been +5 speed stats, correct? and then the pokerus should have doubled it to ten. But when i leveled it, marshtomp only had +4 speed stats so i'm really confused. help?