From DD-WRT Wiki

First aid:

There is a command for configuring VLANs in busybox: vconfig

BusyBox v1.00 (2005.04.16-19:38+0000) multi-call binary
Usage: vconfig COMMAND [OPTIONS] ...
vconfig lets you create and remove virtual ethernet devices.
Options:
       add             [interface-name] [vlan_id]
       rem             [vlan-name]
       set_flag        [interface-name] [flag-num]       [0 | 1]
       set_egress_map  [vlan-name]      [skb_priority]   [vlan_qos]
       set_ingress_map [vlan-name]      [skb_priority]   [vlan_qos]
       set_name_type   [name-type]

Easy step-by-step instructions for putting port 4 on it's own subnet using VLANs can be found at Linksysinfo and Geek Pages

[edit] References

DD-WRT Device Internal Mapping (vlan0, vlan1, eth0, eth1 etc)

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