I have had good luck with the "HB Tune" at www.hbtune.com. It fits the DROID 4 and instead of wearing it on my arm, it fits in my hand. The hand idea was a little different at first, but I got used to it pretty quickly and now prefer it to my arm.
Pretty much hbtune and amazon are the only options for finding armbands for this phone. Not even Verizon has one. But the hbtune is out for me, as I would need both my hands for lifting weights, still nice to have some options though