This is going to over the next 12 months or so be quite a struggle for editors to come up with ways of explaining why inserting SD footage into a 4 or 5k finished piece is not such a good idea.
Producers please take a good look at the image above, it shows the comparative frame sizes ranging from 480p Low res up to 5k. That small orange block in the corner is SD, thats the comparative size of the footage versus what has been generated in 5k. Therefore, unless you want the SD footage to roughly be the size of a postage stamp, it will have to be scaled up. To try and scale up an SD image to the size of 5k, it will mean a scaling of over 1000%. What that means is the 480 pixels have to be scaled to reach around 5200 for 5k. Each pixel will be 1000 times bigger then it previously was. This will render the footage a big pixelated mess that you really cannot tell what it once was.
4k or 5k and SD do not go together. File footage in a 4k finished piece is not a good idea. Please rethink.