As we continue along with 5SK, we'll be developing training aids specific to each Key. One that we saw at the show and were able to test out just today is the Perfect Connextion for this Key #4 stuff.
Just position the Perfect Connextion above your elbows so that the ends are fairly close together (larger chested or broader shouldered folks can use a wider setting). The "loops" bend easily so if you pull your elbows apart you'll feel it, but the aid will not prevent you from doing it by restraint. This aid is also great for people who complete their backswings not by turning but rather by lifting their arms and pulling their elbows apart a little.
On the widest setting (which will push your elbows apart), it can help you by forcing you to actively PUSH your elbows closer together.
So one training aid, two Key #4 uses.
We have a few Mediums at the Academy and they work for both Dave (fairly broad chested) and myself, so a Medium should be pretty good. Click the links above or the picture below to order with 10% off.

P.S. The VisionTrack is also, of course, a great Key #4 training aid, but it doesn't relate to this particular topic about the right elbow position.