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Hi Grant.
They all do the same thing, track/guide in equatorial mode. The difference is basically the higher the number the greater the load carrying capacity, the greater the tracking accuracy, the easier it is to attach electronic tracking/guiding and goto, I think 5 comes standard with tracking/guiding now? (I have an EQ4 which doesn't come standard with tracking/guiding, but I have now outfitted it with an Right Ascension drive. ) Also the higher the number the more expensive.
What you get will depend on a few factors. Are you only doing visual work or do you want to dabble in astrophotography.
For visual work with a small refractor you can get away with an eq3 or 4, which would allow you to do some very simple astrophotography. Basically moon and planets. Maybe some piggyback photos if your really really patient and are a glutton for frustration.
For visual work with a smallish (6" to 8" f/6 ish) newtonian, wouldn't even think below eq4 or 5. Even eq 4 would be very frustrating trying to focus without the image moving all around the place. Its not so much the weight it the length of the scope that will flex the declination axis in particular. (well that's been my experience anyway with moderately long scopes on this size mount.)
If your thinking photography. To be moderately successful no less than EQ5.
The thing you need to remember is that the more stable you need your mount the higher the number you need to go. Visually you can get away with a bit of a wobble as long as your not viewing at high magnification. If your imaging, no wobbles.
Hope this helps
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