Now some 4:3 CRT TVs, especially the later models made when 16:9 CRTs were also a thing, could take a 16:9 anamorphic analogue signal and would then display it as 16:9 with letterbox. Assuming yours is doing that, then I guess your Roku could well be putting out an anamorphic 16:9 picture as a matter of course rather than anything else.
Now, going by my Streaming Stick+, Rokus take a 4:3 video and place black bars on the left and right and then put it out at 16:9. Your TV would then see a 16:9 image which it would likely show in a letter box with black bars at the top and bottom. So, you get side bars added by the Roku and top/bottom bars added by the TV resulting in a mess. On top of that, Youtube uploaders and Youtube themselves might well decide to upload a 4:3 as 16:9 and insert their own bars or may just stretch it and upload because people are strange.
I don't think anything much is designed to put out a 4:3 TV signal anymore. Even if it is, I would expect that functionality to be poorly tested and a bit ropey. I know for sure that source material that appears 4:3 will often be 16:9 with black pillars inserted into the encoding anyway, and can be mucked about with in many creative ways by Youtube 'experts'. It's kinda like having a problem with MySpace and posting about it on a forum asking for advice. The advice will be that it may just be time to move on, in this case over to 16:9 before someone comes up with the idea of High Definition or some other modern craziness...
