Hi and thanks for the replies. Doublet isn't absolutely a requirement, I just noticed recommended doublets at 100mm tended to be long f/9 tubes an wondered if there were good doublets with similar dimensions to triplets available in Aus. I had thought perhaps WO didn't have a 4" since a couple of online shops I had browsed seemed to skip from the ~80mm up to their 132mm models so that is a good option to consider.
Aside from that and keeping an eye on used items, in browsing around I have noticed some options ranging from FPL53 102mm doublets to a few fpl51 or equivalent triplet 115mm telescopes:
4" FPL53 doublet
https://www.altairastro.com/starwave...ctor-459-p.asp
4.5" FPL51 triplet
https://www.altairastro.com/ts-optic...or-10171-p.asp
4" FPL53 triplet
https://www.altairastro.com/altair-w...2715-452-p.asp
4.5" Japanese glass triplet
https://www.altairastro.com/altair-w...-apo-453-p.asp
The 115s are probably about the limit I'd consider in physical size and would be less likely to spring for the Wave series triplet but I wonder if any of them stand out as better value than the others. It looks like some Astro Tech, Altair and TS units are basically interchangable. Also it's curious that the most expensive of these, the Wave series 115 has "Japanese glass" but they don't boast what type.
Another model from Teleskop Service is this Tecnosky which seems almost identical to the TS Optics 102mm doublet except maybe more backfocus if that's useful to me
https://www.teleskop-express.de/shop...mat---OTA.html
I also read through this post re: Apo, doublets and glass types:
https://www.iceinspace.com.au/forum/...d.php?t=173985
It refers to "fast" scopes being harder to make optically high quality. I'm aware the various 100mm f/9 doublets are reputed to be good in part because they're f/9 but what is considered fast in this context and possibly a negative? Would f/6 be an example of too much compromise e.g. this one?
https://www.teleskop-express.de/shop...h-Fokuser.html
Thanks again for your input.