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Hi John, and welcome (back!) I can't comment on any quality differences between the two eyepieces in question, but in my 8" collapsible dob even a 6mm EP is starting to push the limit on planetary viewing in a suburban setting with a 200x magnification. I've tried higher than that, but any increase in magnification is more than offset by some pretty unappealing blurriness. If you can get yourself to a dark site the seeing might be OK with a 4mm, but I'd have my doubts, and even the 5mm might be losing too much clarity. Just my $0.02 worth. Good luck.
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