I just got back inside after taking my new (to me) 4mm Radian for a test flight. After my experience with the 6mm Radian in December, I knew I would "need" a 4mm, too. (I'm using the upcoming Mars opposition as a justification for all sorts of "needs" :-)
I only had a little bit of time, and it's quite cold out, but I wanted to compare the new Radian with my Nagler Zoom to see if it was really worth the money.
It was a typical winter sky here tonight: Very clear, but with only average seeing. The moon was bright, and the ground snow-covered, which made it very bright outside. I carried only my Oracle, the Nagler Zoom, and the 4mm Radian (140x in the Oracle). Swapping between the two eyepieces, I found that, just as with my 6mm, the Radian has a slight edge over the zoom in terms of contrast. On Jupiter, the polar bands were more defined in the Radian. When viewing Saturn, the Radian showed more clearly the difference in brightness between the A & B rings, and showed more banding on the planet than the zoom. Performance on the moon was not noticably different, nor was the view of the trapezium. (I must note here that the view of the Alpine Valley was gorgeous tonight--I think the timing was just perfect for that region.)
The comparison between the Zoom and the Radians is an interesting one because of the tradeoffs between the two: I love the extra contrast and field of view in the Radians, but I also love the ability to zoom in and out on an object. There's nothing like zooming in on a particular lunar detail and then backing out again to put the feature "in context", or adjusting the magnification of a planet to perfectly match the seeing.
Anyway, after 45 minutes in the snow & cold, I came in quite satisfied with my purchase. If I could have only one eyepiece, it would be the Nagler zoom, but the extra money for the Radian was well-spent.
Bring on Mars!
Neil