It would help if you gave an idea of how/where you want to use it — I presume the idea is to be able to access your mp3 collection without running down your S4's battery? Or is it for jogging/gym use? (i.e. something small, light and shock/splashproof, as opposed to huge capacity)
And yes, it's easy to transfer from the S4, depending on what you go for.
But what exactly do you want bluetooth for — connecting to your car stereo or something? (because there are easier/better ways...)