Did you actually go through the "assign network interfaces" process to assign LAN/WAN/OPT1/OPT2 functions to your 4 fxp NICs? The sis0/sis1 stuff should then be overwritten and never appear again.
Fine, thanks. Consider this one solved.
I was unfortunate to go through a Soekris setup first, and there the NICs are identified properly.
The 'Quick Start Guide' says: "Although this example shows sis0 and sis1 as the two Ethernet interfaces these names depend on the Ethernet cards that are installed." That had mislead me. These names do NOT depend on the cards installed. The sentence as such should be changed.
The 'Quick Start Guide' should point out clearly, that one has to set the interfaces. No 'plug-and-play'.
Uwe