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It depends on the APO - some like the TV127is and Pentaxes which are designed to be used primarily for imaging produce a very flat field without correction, but most others need a field flattener to get it flat right to the edges - I know there are flatteners for the FLT110, TEC, TMB and the Tak scopes for example.
There are a large number of options in this area, you can get scopes that are designed for imaging, scopes that are designed for visual and scopes that are good for both. If you want good for both then you are probably looking at scopes in the f/6.5-7ish range. You would generally expect a triplet APO to be better for CCD imaging because it has a wider range of correction across the spectrum than a doublet.
I have an FLT132 which is an f/7 triplet FPL-53 APO, and I know some people use it to produce very high quality CCD images, but I only use mine for visual and webcam imaging so I can't really add much more. For visual a good 5" is definitely superior to any 4" but for CCD imaging its more complicated.
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