Null modem cable hooked to a PC? *snort* Hey, it's less than 100 Euro. =P
If you have another PC (preferably with Linux, but you can do it with Windows too) nearby, that can work pretty well actually...
Only thing I'm not sure about is what speed the 4000 will support on the serial port..
If it's limited to 19,200 (that's about where my 500 maxes) or 38,400, then you're talking dial up speeds.. If you can get into the 100's of K, that's not bad bad for a single connection. Not sure what the 4000 port can pull tho.
Of course, ethernet is better.. ;-)
desiv