From macosrumors Feb 6th:

Next-generation Powerbook G4 details: A modest update to Apple's pro laptops, to bridge the considerable gap between the current models and the Powerbook G5 that will be introduced in the late third quarter, is expected early this spring. The update will be almost entirely in the specifications list and will not introduce any major enclosure or architectural changes.

Details include PowerPC 7457 G4 processors at 1.25, 1.33, and 1.467 or possibly 1.533GHz. There may be 1MB DDR backside L3 cache memory on some high-end models, a feature that is missing from the current Powerbooks. The models which now employ the Radeon 9600 Mobility will probably be upgraded to the Radeon 9700 Mobility, offering a 30% boost to 3D performance along with reduced power consumption and operating temperatures. Larger hard drives and high-performance disk options (5400 and 7200RPM) are expected to round out the update. It is widely expected to be announced with little fanfare, but should keep new buyers interested until the PBG5 drops in August or September at 1.6-2.0GHZ.