What will happen if i take 4 of the .5mg Xanax's?

It really just depends on the MG of those four Xanax you took. If they were .5's, not much will really happen. You'll get tired, maybe a little wobbly while walking. At 1MG, you'd be taking 4MG of Xanax would again make you extremely tired. I'm pretty positive it takes more than just four Xanax to overdose. I'm seveteen as well, I take Xanax; 1MG. So I know this from experience. :)