Back when it seemed like the What If pages were going to be updated the consensus was:

http://marvelmasterworksfansite.yuku.com/topic/13978/WHAT-IF-MAPS-Defenders?page=1

Marvel Masterworks: The Defenders Vol. 3 - 240 pgs
Defenders #17-21, Giant-Size #2-4, Marvel Two-In-One #6-7
A little light on regular issues, but three (close to) double-sized Giant-Size help balance it out.

Marvel Masterworks: The Defenders Vol. 4 - 240 pgs
Defenders #22-30, Giant-Size #5, Marvel Treasury Edition #12
Howard the Duck/Defenders treasury added (just forgot to type it in the listing before, it was in the page count)

Marvel Masterworks: The Defenders Vol. 5 - 248 pgs
Defenders #31-41, Annual #1
Headmen/Nebulon. End of Gerber run.

Marvel Masterworks: The Defenders Vol. 6 - 288 pgs
Defenders #42-56, FOOM #19 (covers)
Full Keith Giffen run.


I believe the Treasury is more appropriate in Vol. 5. The MTU issues (#33-35 IIRC) that were included in the Essential could help bulk up Vol. 4 if the Treasury goes to Vol. 5. MSH #18 with the 1st Guardians of the Galaxy was mentioned as a possible addition to Vol. 4. I don't like that as much as the MTU issues, but I suppose it could work.

I'm really looking forward to the Gerber run. It doesn't get the credit its due as a great Marvel run. 

By the way, whatever happened with the What If updates? There were a lot of maps knocked around a year and a half ago, but no actual updates.