I like jumping 10 times more now that I've shortened mine(-2 burgard swingarm). When/if my motor goes to shit I plan on building it pretty much the same as yours...4mm longrod, then I'll see if I change my mind. But I doubt it. The -2 arm is awesome, I was weary about it at first being primarily a duner myself....but I like it even more! Shee hooks up awesome & I can actually throw it around in the air, is much more controlable during flight. I'm also running +2+1 LS arms.
I'd say +4 would be peachy though,...I don't know about jumping with a +6 arm--could get a bit hairy. I know 'azsandrat' is running an extended arm & jumps some,...may wait & see what he can tell ya. I've got no experience with + swingarms.