It scares me to think if Jackson plans on making a Silmarillion movie, that thing would be split into at least 5 movies all 4-5 hours long (you want a hard read go read The Silmarillion, that thing is hard to get through).
Yea, Jackson left out some of the best parts of the LotR books (like Tom Bombadil, by far the most awesome character ever created). Give them a read when you get a chance but I warn you the first part of the book is freaking boring. I don't know how many times I fell asleep in the middle of reading it (the only good part was Tom).
I wouldn't be surprised by an extended edition being longer but I doubt it'll be 4 hours long. The extended editions of the LotR movies only added about 30-45 minutes a piece (not to mention all the extra crap (the LotR Blu-ray set has 15 discs! I still haven't attempted to go through them yet (yea I bought them, hated the movies but they were still entertaining)).