Load capacity of 2x4s spaced 16"
I don't think those rafter ties will work for support.  The width of the room isn't what's important so much as the span from support to support.  I can't imagine a garage layout that would have a span short enough to make with 2x4's, even sistered.  You'd also have to have the attachments to the top plate of the walls adequate to support the weight of the floor.  If one end of the span is supported by a beam up the middle of the garage, that probably wasn't sized to support a floor either.  

It's doable (almost everything is) but it isn't as simple as doubling up the 2x4's.