This article shows how to bond (or aggregate) multiple ethernet interfaces into a bond0 interface.
First, install the bonding package. This will give you support for bonding in the /etc/network/interfaces file.
Edit the /etc/network/interfaces file:
auto bond0 iface bond0 inet static address 192.168.0.2 netmask 255.255.255.0 gateway 192.168.0.1 # specify the ethernet interfaces that should be bonded bond-slaves eth0 eth1 eth2 eth3
The keyword is bond-slaves that will make ifup add the slaves to the bond0 interface.