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But the 20mm comes to a fairly sharp focus?
OK, have a good look at the 10mm eyepiece - at the front and back glass surfaces. No gunk, plastic coating or fingerprints all over them? Nothing stuck inside the barrel of the eyepiece - a lump of polystyrene? Look through it by itself at a bright window or light (not the Sun). Nice bright circle of light seen (all out of focus of course)?
If that all checks out OK, then it may be that air conditions ("seeing", "transparency") weren't good enough to get a sharp image with the 10mm.
Try the scope during the day - looking at distant trees or buildings - and see what you learn about the eyepieces. The air will be less stable (especially you have it hot over there now?) but the image is not moving (like the Moon/stars) and is full of detail across the entire field of view.
Try the 4mm eyepiece during the day as well - I suspect you will find it will be a poor performer.
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