For Soekris Net4801 you should use the WRAP version. The serial speed is 38400 bps and not 19200 (the default for then Net4801).
To install the downloaded file on a 512MB Compactflash you have to uncompress the file with gunzip and then use dd unix utility to write it on the device on which the CF is connected. In any case, read other posts on this forum for greater details.
Regards
Fulvio