So you are the Head of D&D at Wizards. Fourth edition has not been the massive success that was hoped for with many fans sticking to 3.5 or worse, going to 4e and switching back. D&D Insider was a year late and is still missing major features. Your business model is based on selling lots of $20 rule books but the economy has tanked and your core market has less money to spend.
What to do?
Let us assume that you want to ensure the long-term survival of the D&D brand and profits and you are not chasing short-term sales in return for a big fat bonus.
The best possible thing that you can do is give away ALL the 4e books as free, black & white only, PDFs. The core books and the supplementary rule books (but not the modules or similar). What would happen if you did this?
1. Lots of people who currently cannot afford 4e rules now have access to them and can play as equals along side their richer friends. This will be especially true amongst the sub-18 market who represent the long-term value of the D&D brand.
2. Sales of the books with quality content will increase. When players read a good rule book / supplement as a PDF they will buy the paper version just to have the connivence and feel of a full colour, bound copy. This encourages WotC to produce quality books.
The long term success of games like D&D is people playing the game. By making it freely available there will be more people playing it. More people playing means more people interested in buying the hardback, the modules, the miniatures, the fiction, the films, the t-shirt etcetera.
Making the rules, all the rules, of 4e free would give the game a massive shot in the arm in the short-term and greatly increase the number of people who will be interested in 5e when it inevitably comes out.