For Vol. 3: They will surely put issue #21 into this one. It is otherwise a bit light on actual Defenders issues. I would honestly not rule out issues 22-25 (the "Sons of the Serpent" story) going in here too as they are clearly being quite generous with page counts for this series. And that would still be a little shorter than Vol. 2. So I go with 17-25 + G.S. 2-4 + Marvel-Two-in-One 6-7. I don't see a reason for Marvel Team-Up #27 as it lacks any direct tie-in to the Defenders stories.

For Vol. 4: So if that's Vol. 3, than Vol. 4 would be biggest and stongest of all (Hulk SMASH!), what with issues 26-40 + G.S. #5 + the first Annual (issues 40 & Annual #1 would be needed to wrap up the whole Headmen/Nebulon saga). They miiiiight even kick in issue #41 to wrap up Gerber's run for an epic, 350+ page tome.

I would normally not expect Masterworks this big, but hey...it's the only way I can explain why they would have shoved issues 15-16 in Vol. 2 when it would have been quite nice ending with ish #14.