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Hi Revan,
Focal length in a Newtonian/dob telescope is the distance from the surface of the main mirror in a reflecting telescope to the point of focus. You can measure the distance from the mirror to the diagonal and add the distance from the diagonal to the focal point. In your 4SE, the folding of the optics magnifies the image to create a longer equivalent focal length and can't be measured linearly.
Experienced observers will tell you that it's not all about the magnification, you are referring to magnification as zoom. Mirror diameter does not affect magnification. If you put the same eyepiece in two telescopes with the same focal length, the magnification will be exactly the same in both telescopes.
The resolution is proportional to the diameter of the telescope mirror or lens. A 4" scope can theoretically resolve 1.1 seconds of arc, a 10" scope can resolve 0.45 seconds of arc. A second of arc is 1/3600th of a degree.
Atmospheric turbulence (seeing) limits the highest magnification you can use.
Large deep sky objects are sometimes best observed at low magnifications. The dilation of your pupil limits the lowest magnification you can use and that pupil dilation decreases with age. If you use a magnification too low, not all of the light exiting the scope will enter your eye and you will see objects of similar brightness to that seen through a smaller aperture instrument.
Cheers
Joe
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