So say John wanted to get to Dave's world, he'd have to use his client portal to get to Jade's world and find a client portal on her world? Would he be using one of Jade's green portals because it's her client player, or would it be a blue portal somewhere on Jade's planet?

He could do that or he could just go through his 4th gate.  And I’m not sure about the color of the portal but it’s most likely that all the portals on a planet are the same color.