First, may I ask what your slingbox model number is?
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？
Slingbox & Slingbox channel on Roku
I have used a Roku 3 and connected to my Slingbox located in another state using the Slingbox Roku Channel. It has worked wonderfully.
About 6 weeks ago I purchase a Roku 4 to replace my Roku 3 which I was going to move to a different TV. My case number is : 1041713
The 3 works fine but the Roku 4 has a display problem - it crops the picture (bars on each side). I have contacted Slingbox several times and supposedly there engineers are looking into the problem.
I am not sure Slingbox even has a Roku 4 to replicate the problem. I have given their support people access to my slingbox and they don't have a Roku 4 so they didn't have the problem.
I have contacted Roku and asked them to talk with Slingbox....
I am posting this problem here in hope that others see it. Its not a settings issue from a user's perspective, its a problem with how the channel interacts with the Roku 4 box.