Should i play with a pick or my fingers?

i recently bought a new electric bass (my first), a four string Schecter Omen 4, used, with strings which are about 1 year old. i've been playing the bass for about 4 months, and i'm pretty good, but i can only play with my index and middle fingers. is that normal? i've tried using my ring and pinky finger but they're too weak. should i play with a pick or just two fingers?