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Hi Afro - I started with a 4" reflector that only used 0.96" eyepieces. My next step from there was a 10" dob - and what a difference it was - chalk and cheese. I stayed with the 10" dob for years.
Depending on your budget, it might be worth looking for an 8" - 10" dob that comes with a couple of eyepieces in the deal. I guess you need to at least keep in mind that you probably will, in the future, move up to a larger / more advanced scope. That being the case, just keep in mind what sort of focal length eyepieces you grab for your current scope as they may produce significantly different magnifications on your next scope.
To determine power in a telescope, divide the focal length of the telescope by the focal length of the eyepiece. Your current scope is 900mm FL, so a 5mm eyepiece (for example) would produce 900 / 5 = 180x magnification. If you moved to, say, a 10" F5.6 scope later on (FL of 1422mm), that same eyepiece would give you 284x.
Most sources will note that the maximum that you would get out of a reflector on an exceptional night is about 50x per inch of aperture. On your 4", that would be around 200x, and on a 10" around 500x. However, I would expect that most of your observing would be well below those limits.
Anyway, hopefully that info may assist you a little with choosing the FL of any new eyepieces to suit you into the future.
All the very best with your purchasing Afro, and welcome to a new and endlessly interesting hobby!
Dean
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