You are right, some eye pieces work a lot better in faster scopes than others, that's about as much help I have in that area
At F/4 you may want to get a coma corrector. Some learn to live with the coma and outer field distortion, others swear by correctors in fast newts
I'd suggest something along the lines of a 20mm and 10mm with a good quality 3x barlow for planetary observing.