I’m guessing your drive is formatted fat32….. you can find out by going to my computer and right clicking on the hard drive and going to properties…. fat32 file systems have a file size limit of 4Gb plus if you are rendering to AVI I think they also cap at 4Gb…. I’d suggest you reformat at least one of your drives to NTFS – it’s a much better file system when working with large volumes and large files….
alternately you could try rendering out an image sequence (such as TARGA), this way there will be not be one single file…
you can also find out more info by searching google or the COW archives for “FAT32” or “4Gb limit”
hope this helps you