Johnny, the 4mm is not darkening the image. You are asking a lot from any eyepiece to magnify so much, and every eyepiece of that focal length only has so much light from a small field that it can use. The image is 'dark' because that's all that's available from that area with that scope. I have exactly the same situation with all of my high power eyepieces, and something I have encountered always. When I'm doing a lunar sketch, and I chance upon a very stable night when I can really up the magnification to my 5mm, it always strikes me just how dark the image has become from the lower magnifications I was just using before. I am using a 8" SCT too. As I don't get to use the 5mm very often at all, I next to always first think "What the heck have I done here!..." Then I remember what eyepiece I'm using,
It is interesting to note that you find eye placement is more finicky with it. Not something I particularly noticed with my ones, but I might just be more accustomed to them.
With respects to a 13mm, good eyepieces in that focal length that don't cost the earth, perform well in a fast Newtonian & are easy to use eye relief wise are desperately short in numbers. The one I am happy to recommend is the GSO 15mm Superview. Bintel has the same one with their badge on it. I have this EP too. Sure there are better eyepieces than this one, for the price I don't think it can be beat. If you are price sensitive, then this modest little EP is a gem!
While I may not use mine too often, it is one EP that I won't be giving up.
I've just had another look at the Astro Pete's site. I see they carry the Saxon 'Super Wide Angle' line of eyepieces. These are rebadged Baader Hyperions and are designed to be used with SCT's and Mak's - they are NOT a good optical match for Newtonians! Brian mentioned that he got a good image with these in refractors - this is so because the shape of the focal plane that refractors, SCT's and Mak's all produce is the same concave shape. Newtonians produce a convex shaped focal plane, so the Hyperions will not respond well to this. The only exception to this is the 5mm, which is terrific in my fast Newtonians and my SCT. NOTE: I am not writing off the Hyperions, not by the least. You just need to be aware that not all eyepiece designs will work equally well in all scope designs. If you have an SCT or a Mak, Hyperions, and their variants, are really good!
This lesson with the Hyperions cost me a bomb to learn,
, but it opened my eyes to the poorly understood aspect of matching optics with optics, and that different EPs work best in certain scopes - even in the one product line there can be significant differences between individual models! The Pentax XW line is one of these!