Lewis, I'd be the first to say ortho, but at 4mm their eye relief is extremely short, and below that you are pretty much limited to non Abbe designs like the Nagler Zoom, Vixen HR or the hard to get Pentax XO. The latter two I have, and they work extremely well in my fracs, but they are pricey. A good extender might be a better choice, since it can also be used in your imaging if you are after more power there.
Edit: extender as in Powermate or the ES version of it
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