It is an interesting conundrum! From reading many many reviews of different telescopes (haven’t used many but I’ve read a lot
) I would strongly advise against going below F/4 unless you’re willing to soend the big bucks. Just look at the mechanical issues that Paul has been having with his GSO F/4. Optically they’re good but mechanically they are made to a price point that doesn’t allow the stiffness and tolerances required.
The next consideration is whether you want a complement or a replacement for your 4”. If you want a complement I’d suggest going down the path of maybe a CFF 10” RC and getting a KAF-16200 as Rick suggests. This would give you a wide field setup of 2”/pixel with the 4” and a DSO setup of 0.5”/pixel with the 10”.
If you’ve only got one mount and going to use one telescope at a time, you may as well get a single camera to make the best use of most systems
If you don’t mind a replacement, getting a 5-5.5” refractor is a good choice. If you wanted different resolutions you could use something like a QHY183M with its 2.4 micron pixels and a KAF-16200/16803 for wider fields
