The difference between the XBR4 and XBR5 series sets appears to be in the finish alone. Both series include sets in screen sizes of 40", 46"' and 52", and both series include Sony's "floating glass" frame design, which enhances the illusion that the image is floating in space. The XBR5 series comes only in classic piano black, while the XBR4 series features the ability to swap put the standard black bezel for any one of eight optional colors to match your decor: Scarlet Red, Pacific Blue, Arctic White, Sienna Brown, Titanium Silver, Midnight Black, Rose Metallic, and Champagne Gold.
Technically, both XBR series sets include all the features listed for the W Series but stand apart from the non-XBR series sets with Sony's Motionflow 120Hz high frame rate technology and more advanced video processing based on BRAVIA Engine Pro circuitry and DRC v2.5.
120Hz refresh rates are becoming common with premium LCD displays, and is touted as greatly eliminating motion blur. What's fascinating is that 120Hz allows essentially a double frame rate for 1080p/60 signals. But 120hz is also a direct multiple of film's 24fps frame rate- movies on Blu-ray Disc and HD DVD are encoded at 1080p/24 and more and more players are allowing output at that native rate. It appears that Sony's adjustable Motionflow modes will display a 1080p/24 signal at a direct 5x frame rate of 120Hz, which would remove judder and any hint of flicker in the image. The other 120Hz displays we're aware of either don't accept 1080p/24 signals, or convert them to 1080p/60 before converting to 120Hz.
