Well, sqm-scripts will first need to create a new version that incorporates that fix before the openwrt package can be updated, s this will take a while.
Sure, sorry for that.
I believe that issueing the following command from your router's command line should do the trick:
echo '[ "$ACTION" = ifdown ] && /etc/init.d/sqm enabled && /usr/lib/sqm/run.sh stop ${DEVICE}' >> /etc/hotplug.d/iface/11-sqm
Then do "cat /etc/hotplug.d/iface/11-sqm" and look at the output (and/or post it here)
If you do try "ifdown wan" followed by "tc -s qdisc" (after a short while there should be no cake/htb information left for the ifb4${WAN}), then "ifup wan" should get you back to a working sqm instance (confirm with "tc -s qdisc" where you should expect one cake or htb instances on your wan interface and the matching ifb interface; but you might need to give that some time as for example pppoe might take a few seconds to actually give you an hotplug event after issuing the ifup command.
Let me know whether that works.
Best Regards