I thought "every" Roku could do SD mode. Maybe Roku 3 is the only one that doesn't?
The Roku 3, Roku Streaming Stick, and Roku TVs are all limited to HD resolutions. I'm not sure about the new Roku 2, but I suspect it might be as well.
This question was posted when the Platform Secret Screen feature supported the ability to fix the vertically stretched image, and it was called “Select all resolutions” or “Enable selecting all resolutions”.
Anyways, there is no way to have a 4:3 SD resolution on an HDMI only Roku model, not even when using the Bleiden HDMI to RCA converter, as that resolution seems to have been unsupported from the Platform Secret Screen feature, and the Bleiden HDMI to RCA converter might literally only convert the signal to the analog RCA outputs.