I have lost the screws for my digital Piano, and would like to buy new ones from Home Depot or a local hardware store. The manual has some info, but it seems that I also need the tread size to find the proper screw. Any ideas how I can find the right screws?
Some of the screws appear to have threads for wood (or plastic). The actual thread pitch isn't terribly important, so just buy screws of the correct size and with suitable heads.
Screw E is obviously a machine screw. 4mm screws typically have a standard thread pitch (0.70mm), so what you find should work. If there are multiple options, buy both and try them.
In case some of the 5mm screws are actually machine thread, common pitch is 0.80mm. Again, trial and error. They aren't usually expensive, and if you're careful with the packaging you can return leftovers.
As isherwood noted the plastic/wood screws should be more-or-less interchangeable for anything of the right diameter and length. Make you you get a screw with that washer head, though, that is required for helping to prevent pull through. Also, be sure to get the correct length - too short and it may not go far enough in to hold the other piece of plastic - too long and it could punch through the back of where it's supposed to stop and could hit something important like a circuit board or the point could stick out through a leg and catch your pants or skin.
For the machine screws, length, diameter and thread-pitch are very important. Remove an additional screw of the type that's missing (support the piano, if it's already assembled, to make sure the stand doesn't collapse with an additional screw missing) and take it with you to the store. Most big-box home improvement stores will have a screw thread sizer in the fastener aisle. Simply screw yours into the various nuts supplied until you find the one that it goes into very easily and without effort - that will be the correct pitch. If you have to force it at all, then you don't have the right thread pitch. Then all you need to do is find a small package of screws with that pitch and the correct length.
If you have lost all the screws, then buy a variety pack of machine and wood/plastic screws. You know the diameter and length (4 x 14mm is 4mm diameter and 14mm length), so those are easy. Just get something that has the right diameter/length in machine screws with a few different thread pitches. Try them until you find the right pitch to match up with the machine screw holes in your piano.
Keep the extra screws. Everyone needs a jar of random screws for future projects just like this. As a matter of fact, if you'd purchased a pack of screws for a previous project, some of the leftovers may have been just perfect for this one! ;)