Thank you as always for your comment @littlenewthings. You are correct in saying that 1/4000 is the shutter speed. It basically means that the cameras shutter stays open 1/4000 of a second (a fraction of a second) allowing you to freeze fast moving subjects. Just know that the longer the shutter is open the more light hits the sensor. That's why when you are taking low light pictures the shutter speed is longer like 1/40 second or even 30 seconds.

I might have to write a blog on it in future! I'll also look out for your photo under the hashtag!