This is a bit of sloppy work on my part. I must have had a collision of hardware revisions in my head when writing this...
SG 105(w) exists in three hardware revisions, Revisions 1 and 2 are externally identical (they have slightly different components). Revision 3 is different externally; it has an SFP port, which messes up the order of port detection by OpenWrt. Here are the two revisions together:
Revision 1 / 2 is straightforward; the order in which OpenWrt detects ports is the same order in which they are labeled. On Revision 3, OpenWrt detects the SFP port first and gives it the designation eth0; the same designation is then given to the RJ-45 twin of the SFP port, which is the rightmost of RJ-45 ports. The remaining ports are designated eth1, eth2, and eth3 left to right.
Then, whatever is designated eth0 becomes LAN, and whatever gets the eth1 designation becomes WAN. The remaining ports, as @slh just explained (thank you!), are left unconfigured for the time being, but can be easily added into the LAN bridge.
So I have fallen into a habit of going straight to /etc/config/network first thing after installation and editing it to be consistent with whatever labels I have on the outside. For Revisions 1 and 2, eth1 would be WAN and the remaining three ports would be members of br-lan. For Revision 3, eth2 would be WAN and the remaining three ports would be members of br-lan.
Once you're done editing the configuration, you can reboot the device or restart the network service (service network restart on the command line) to apply changes.
Hope this helps.