Hi,
When I bought my Epson P-2000, the largest SD card supported by the then current firmware was 1GB. At the time, I spoke to Epson about this and they implied that the device was only OK with SD cards up to 512MB and there were no plans to provide support for larger cards. However, as you say, not that long after that they released firmware 2.51 which supports cards up to 2GB (it does). So, until Epson pulls its corporate finger out again (and you have no way as an individual of influencing this), your 4GB SD card will not be read.