Should I stay in my uni for four years even though I already have 50 credits?
First of all, I know nothing about Biology at all. I can't comment on specifics. Instead, I rather say something about the comment:


  Do you think double-majoring using the available time gives me a significant advantage in my admission? Or should I just concentrate on the area which I will study in grad school?


You are far away from grad school yet. Undergrad education is for you to build the foundation of your academic career. You need to use it to broaden your knowledge base. Many courses can be benefit for you. You should consider taking the courses about humanities, fine arts, other sciences (math, chemstry, physics, computer science, etc.) and writing, etc.etc. Just don't limit yourself to a specific field. You won't know you're interested in something until you learn it.

The above is from the bottom of my heart. I wish someone would have told me this when I was an undergrad student.