Please, why have original people on Avatar 4 fingers and DNA people on Avatar have 5 fingers?

Well as it alisema in the beginning of the movie, Avatars are fused with human DNA. Thus, the Avatars are bound to have human similarities such as a fifth finger and eyebrows.

Navi are born with 4 fingers humans in Avatar body has DNA from human body so he have 5 fingers , other eyes than Navi etc.

oh, the avatars have five fingers? i didn't notice.

The original people au Na'vi, have 4 fingers, yes. The DNA people au Avatars, have 5 fingers because human DNA is mixed with the Na'vi DNA. Human DNA makes avatars have 5 fingers.