I'm pretty sure Time Crisis 4 runs on the System 258 board
(not 256, as there is no such thing AFAIK). the initial reports from a few weeks ago said System 258. the same board that runs Tekken 5 and Tekken 5 DR.
it makes no sense for Namco to produce another marginally faster board based on PS2 technology, let alone two more, beyond the System 258 from 2004, because a PS3 based board should be right around the corner.
edit: it is possible that the System 258 board that powers Tekken 5, Tekken 5 DR is actually called System 256. regardless, this is a board from 2004. and I dont think Time Crisis 4 uses anything beyond this. I think all the recent and new Namco arcade games are running on the System 258/256 board, and that there are no new boards beyond that until the PS3 based arcade boards arrive. maybe this will be proven to be incorrect though. we'll see. but Time Crisis 4 certainly doesnt look like its using anything beyond the circa 2004 board that powered Tekken 5.