Piccolo looks better with five fingers than four.

[Scott]

He looks deformed with just four. I know it's to make him look less human but nah, it's unnecessary. Just give him five fingers famalam

 

[Warmmedown]

I admit I never even knew he only has four fingers, including the thumb. In fact I still don't know....well it turns out you're right. Actually on google images I see he sometimes has five. I guess four is the manga.

 

[Scott]

Yes, he has five fingers in the anime and four in the manga.

There are actually some panels where Toriyama goofed and gave him five fingers.