Toshiba sucks hard core. As Dan pointed out, if you attempt to install Vista on a Toshiba Tecra M4, you'll either be greeted with a warning, or you'll crash once you get Vista installed. The problem? Vista doesn't run properly without a BIOS update. Toshiba has one, but because they're stupid, they haven't put it out to their customers yet. They haven't even informed us as to why there is a problem, and advised us on workarounds until the update is available! They're content instead with frustrating, annoying, and pissing off their customers instead! (I pointed at an Nvidia rant there because apparently the BIOS issue is one of the reasons the video card hasn't been stable)
Right now the update is only available for Microsoft employees. So if you want it, please innundate Toshiba with support calls, and tell them you deserve the BIOS update! In the meantime, I would recommend waiting until it is out before installing it on your machine.