All M4 machine screws have the same 4mm thread diameter to fit the matching nut. You will also need to match the thread count (per inch or whatever). Easiest way to do that is in a hardware store with loose stock. Take the screw you have and the one you think you need and press them together side by side with heads not contacting at opposite ends. If the threads do not mesh perfectly keep looking. They may also have a tester you can use to find the thread count. Then you will know what you are looking for at your next stop.

M means metric and 4 is the diameter of the screw in millimeters. Just like SAE screws, metric screws are available in both fine and coarse thread pitches. The pitch is the distance between the valleys (or peaks). The correct designation would be M4ÃÂ.7. If Wikipedia is correct, M4 screws only seem to come in a coarse pitch, so any M4 screw should work for you.

Thank you both so much!

better yet. Take the knob to the hardware store and ask the clerk for a longer screw that will mate with it.

We ran into this all the time when I worked at Ace Hardware. It should be just a "standard" M4 that you need. However, it is common (and highly suggested) to bring both the knob and original screw in. You should be in and out of the store in just a few minutes.

And fyi for everyone. M4 screws do come in multiple thread pitches, Some are just harder to find than others.