Up to isomorphism, how many additive abelian groups G of order 16 have the property that x+x+x+x=0 for each x in G?

I don't know where to start with this problem. I don't know how to figure how many additive abelian groups of order 16 there are to even begin narrowing it down to those with the given property. What property limits the number of abelian groups?