Well is mostly comes down to fit. My wife road a couple Specialized Ruby's but they just don't fit her nearly as well as a Madone. That said her 5.1 WSD has been on order forever and still won't be shipped until maybe April - driving us nuts. As for the 4.7, it does not replace the 5.1, but gives Trek a slightly lower price point. There are also some differences. It's an imported frame and not made in the US. It also uses Trek TCT carbon rather then the OCLV carbon used in a couple different weights for the US produced bikes. It also uses a traditional seat post, rather the the new design on the US built Madones. For the price difference, I still think the 5.1 is the way to go, but purchase should be made based first on fit.
All Specialized bikes are imported, so I don't know about better built? The zertz inserts seem like a gimmick to me. Trek also has a excellent warranty, and I love my '06 Madone.
Go with fit first, but the Madones are awesome bikes. Good Luck.