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There's some argument for sampling at 3x the possible resolution of the system if you don't want to leave any detail on the table, but like everything it's a trade-off.
Additionally, the diffraction limit isn't really a hard limit and changes depending on which formula you use. Dawes for instance gives you 1.14" for a 4" scope.
Jon Rista (posts on CloudyNights) uses the following formula to estimate FWHM:
FWHM = SQRT(Seeing^2 + Dawes^2 + ImageScale^2 + GuideRMS^2)
I'm going to make some (reasonable) assumptions and say we have typical seeing of 2", and a guiding RMS of 0.4", which means for your current (native FL) system, we get:
FWHM = SQRT(2^2 + 1.14^2 + 1.07^2 + 0.4^2)
FWHM = 2.56992218"
Keeping everything the same except switching to an (unobstructed) 6" aperture you would improve your resolution as follows:
FWHM = SQRT(2^2 + 0.76^2 + 1.07^2 + 0.4^2)
FWHM = 2.42538657"
And for the sake of illustration, were you to continue to use your 4" scope but go for pixels half the current size, you'd be looking at:
FWHM = SQRT(2^2 + 1.14^2 + 0.535^2 + 0.4^2)
FWHM = 2.39704506"
As you can see, the diffraction limit is not a hard wall, and you actually get (admittedly marginal) increase in resolution by going for even smaller pixels.
My 2c is that a 4" is fine for you, especially given that you primarily shoot nebulae.
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