I have a 4mm GSO EP and it's virtually useless - and I'm one of the few people around who can easily deal with the zero eye relief on the GSO cheapie one
At that point, sure, you've magnified the planet. It's just a shame that now you've got a blurry ball with no more detail than you intended. It's like looking down a road on a very humid day. If anything, the most effective use of a 4mm EP is monitoring the current atmospheric distortion. Even looking at the moon isn't that great.
But, no matter how much anybody tells you (and I was told) - you've got to witness it for yourself
The reality is, dobs really arent' that great for planetary viewing. I'll keep saying this till I'm blue in face on here because sometimes, people seem to miss that important tidbit out. Planets are small. Even through a 12 inch dob and a 4mm EP.
I know next to nothing on any of this subject bar the fact that the views from higher focal length scopes seems to beat the pants off a dob, for viewing planets.