I replaced one of my SD1A203T-GN with 4MP SD1A404XB-GNR (both from Andy).
Definitely a noticeably sharper image in the daytime. Night time performance is a touch less light sensitivity than the 2MP version. It's still a lot better in the dark than a non-starlight, and I can see everything I want in the dark after fine tuning it .You'll notice the decrease in light sensitivity if you go from the 2MP, but if you never used a 2MP version, you would not realize there is a difference Overall, I'm pretty happy with going to 4MP.
Depends how you define better. I've got a bunch of SD1A203T-GN in use and replaced one with a SD1A404XB-GNR. Sure, it's higher resolution, but it wasn't a dramatic difference. I'm not in any rush to upgrade my others, but if I was buying one today I'd probably just go with the SD1A404XB-GNR despite it's night performance not being as good. I'm not a huge IVS user but it has the more advanced IVS and some other things, plus it's newer so presumably it will get firmware updates a bit longer and things like that.
