Which level should I train my Pokemon?

Question

in black and white before the E4 which lvl should I train them?

Answer 1

Level 55 or over. The Elite Four's Pokemon are generally around level 48, although their most powerful Pokemon are higher. So to stay stronger than your opponent, level 55 - 60 should do it.

Of course, it is definitely possible to beat them with Pokemon of similar or lower levels then theirs; although it will be easier with Pokemon about 5 levels stronger.