My slinbox PRO-HD and Roku4 has the exactly same issue(aspect ratio is 4:3)!
I also asked Roku's support center, but they told me this issue belongs to sling media.
After that, I used my firend's slingbox which is slingbox 350 to try with my roku4, it work perfectly! The aspect ratio is 16:9, no black bars on each side of the picture.
I gueesed my PRO-HD is too old, and considering to buy a 350 next time.
Did you solve your aspect ratio problem eventually？