4" is plenty good enough. Just optimize it with a really high performing "planetary" diagonal (i.e., not a dielectric, use a prism for best contrast transfer), good contrast eyepieces, a Powermate over a Barlow if you intend to use longer focal length eyepieces, and a Baader Contrast Booster filter (or a precision ND or Polarizing filter to attenuate the light; or some good single coated optics instead).
I have a 10" Dob, 6" Apo, and 4" Apo. Even when seeing is stable it does not necessarily mean the views will be good as particulate and water vapor levels in the atmosphere can still kill the contrast and make the view look washed out. So the 4" will often do just as well as the 6". The 10" Dob will of course get way more fine details, like the eddies around the GRS and more detailed internal structure in the GRS, but not worth the effort to get the mirror thermally stable enough if it is a scope that goes from inside to outside and the view is many times bright enough to wash out the finer details if the seeing is not letting you get to a small enough exit pupil to tone things down. But the 4" shows wonderful details and is just plain fun also. So even with my 6" Apo, I still take the 4" out way more for planetary and get lots and lots of satisfying views. Don't get me wrong, the 6" is wonderful and gets more planetary details when the conditions allow, but compared to a 4" where it really shines being substantially better is on DSO, particularly galaxies -- galaxies that are a struggle for the 4" are just right there in your face in the 6".
This view was with the 6" Apo. But comparison a few weeks later with the 4" the view was substantially the same, with only the mottled shadings in the southern hemisphere not being evident and the desert regions not having this sandy/grainy textural appearance that the 6" was showing. So the detail loss was not to the major features, but just some of the internal shadings.
In these two views note the difference in contrast the eyepiece can do. The 1st pic is using a singlet sphere eyepiece, whereas the 2nd one is using a wide field that has 8 elements in 4 groups. The more glass, the less contrast (everything else being equal).
This view of Jupiter is with the 4" Apo and binoviewing with 28 RKEs and Barlow. Note that even with a 4" one can see structure inside the GRS.
Edited by BillP, 16 November 2020 - 03:01 PM.