Well, not directly tied together. I think the easiest way to do that would be to have Ultiamtes V3 and V4 focus of diffrent sets of characters, with the big ones covering both. Say Captain America, Iron Man, and Thor are in both, Vision, Flacon, and Bruce Banner/Hulk in V3 (I want Joe Mad on the hulk) and with V4 having Falcon, Wasp, Hank Pym (With his return being in V3, but if you've never read an Ultiamtes than theres no problem there.) and the newly minted Ultimate Black Panther. You wouldn't have to eplain why they're not in either volume, or if you really wanted to there could be a comment about personal leave after the events of V3, wich of course would make people want to read V3 but hardly make it a requirement.