Excellent report Steve
I recall seeing Mike Hosea over on CN saying that these work great at f6, but off axis sharpness is degraded at f5. Apologies to Mike if I got this wrong.
I might get a 4mm for my travel scope as an alternative to vixen lv.
This eyepiece appears to be a direct competitor to the TV Radian and also the pentax XF. Steve how would you characterise the B/TMB comparing to the XF ?