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The rule of thumb for the maximum usable magnification M is 2x the aperture in millimetres. So a 4" = 100mm gives 200 times.
Now the magnification of a telescope and eyepiece combination is given by M=F/f where F is the focal length of the telescope and f the focal length of the eyepiece.
Turning this around, the minimum f is given by F/M which in your case is F/200.
You didn't say what the focal length of the telescope is, but assuming ideal conditions and given that the 10mm works F can not be more than 2000mm, and that the 6mm does not work means it is at least 1200mm.
So for example an 8" SCT - aperture 203mm and focal length 2032mm - supports about 2x203=406 times magnification which you get with a 2032/406=5mm eyepiece.
You can only use these maximum magnifications if the seeing is good.
Andrew
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