At F4 you're definitely going to want a coma corrector, and it gets tricky at relatively short focal lengths too. The TeleVue Paracorr Type 2 is a highly regarded (perhaps the most highly regarded) corrector and even it is documented as introducing curvature at focal lengths below 1200mm.
I suspect this was the cause of poor performance off-axis in my 8" F4.5 when combined with the older Paracorr (and also the TS Optics corrector)... I ended up going down to a tiny sensor (IMX 178) to solve my issues.
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