Conversely I've seen exactly eight worlds over 3g and zero over 4g (in my current readable journal, I know that I saw higher prior to what I'm parsing).I've seen lots of worlds just over 3g and very, very few just over 4g so far, so ≥5g seems right as far as I can tell.
One thing I want to make clear is that I don't intend for Elite Observatory to be used only by the record setters scouring the galaxy for only the nth top placement in a particular category or better. I want a fresh starting bubble skipper to be able to grab it and find something that might make them say "neat!" before they get bored. The built-in checks are meant to maintain a fairly steady stream of things that might be interesting to go look at or land on.
Of course, people that want to find the biggest and best are welcome to use it too, and I'm more than happy to support them in doing so (varonica's dump of new worlds in the largest planet thread was fantastic), just keep in mind that the target audience is a little wider than just that. If you want to tighten the tolerances of what's searched for to remove some of the "cruft" then by all means disable the built-in checks you consider too lenient and add some custom criteria of your own. I provided a >5g one above, and I'll help with creating others if anyone isn't quite sure how to make it work.
edit: Changed phrasing to make it more clear that I do want record setters to be able to use it, just not them alone.