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？
So I've been streaming from a UK-based Slingbox to my local Roku TV for a little while now, and even though the image is stuck in 4:3 aspect ratio, I used to be able to just go into the Roku TV's "advanced picture settings" and change the "picture size" option to "Stretch" to bypass that issue and present the stream in a natural 16:9. However, since about last week, when I go through that setting, it stretches the image as it would were the image a full 16:9, center-cutting the actual screen image. I don't know how this changed, perhaps it was on the UK end, but I have no access to that end so I can only do things remotely. Is there any way I can fix this, or get the aspect ratio (or the workaround) correct?