Crisp views depend as much on seeing as on the telescope. Thus, the high magnification that in theory a larger aperture should allow is rarely attainable and often restricted to below 200x.
However, a larger aperture will gather more light and will give better resolving power. You probably don't need the extra light for the planets but you will for some of the moons e.g. Neptune's moon Triton, which is magnitude 13.5.
An 8" Dob will have twice the resolving power of a 4" refractor. You will pick up finer surface detail in any object that you look at. The extra resolution becomes quite noticeable when looking at the detail within the Homunculus (eta Carinae) with a 30" Dob. You just cannot get this detail in smaller apertures.
Regards, Rob
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