Anyone aware of a spinnerbait that will run straight, less than 5/16 oz, with a single no. 4 Colorado blade?  Not the thin Colorados but high end ones like a Hildebrandt.

Can't think of a company that offers one, but a #4 on a bait that light is going to lay on it's side during anything but a slow crawl. It's just too much lift for that light of a spinner.  You could try altering a 1/4oz. SB by removing its blades and adding a single #4 Colorado.

Your request is pretty specific.  I would suggest just buying a few single-spins of the weights you like and experiment with replacing the blades.  You can get a 5-pack of Hildebrant blades from Barlowstackle.com.

War Eagle makes a quality spinner bait.  They make a finesse spinner bait in 5/16 oz.  Comes in neat colors.   It is a tandem spinner bait that comes in Colorado/Turtle back blade, silver frame or gold frame and I don't know about your local Walmart, but they are on the pegs at mine.  The turtle back blade is very similar to a Colorado blade, if you've got to have a single spin - 20 seconds labor with a set of side cutters and you've got one.