Xen error: Link veth4 is missing

On default Xen has four veths to communicate between bridges and real eths. If one uses a command:

/etc/xen/scripts/network-bridge start netdev=eth4 bridge=xenbr4 vifnum=4

to create bridge xenbr4 and connect it to eth4 then he receives a message with
Link veth4 is missing at the beginning. To fix that situation gateway.auckland.ac.nz:/boot/grup/grub.conf should be changed like this:

kernel /boot/xen-3.gz dom0_mem=262144
module /boot/vmlinuz-2.6.16.33-xen ro root=LABEL=/ netloop.nloopbacks=8
module /boot/initrd.img-2.6.16.33-xen

A number in netloop.nloopbacks parameter is equal to amount of real NICs. Real gateway server shoud be rebooted.