It sounds like you're pushing the magnification too far. You'll get the best contrast on planets with 1.5-2mm exit pupil, best detail (when seeing allows) with 1.0-1.5mm exit pupil, or so. Not knowing what our scope focal length / f-ratio is I cannot say what eyepiece will give you that, but I am sure that a 4mm in a barlow is way too much power. You should be using EPs between about 1 and 2x the f-ratio of your scope, e.g., in an f/5 you'd use a ~5mm for high power (detail), ~10mm for high contrast, ~7mm for best compromise between these. Well made and well set up Newtonians perform very well on planets.
PS. Aperture rules.