According to the hardwarescheme you need to delete port lan4 from interface lan and add corresponding cpu port eth0 it to interface wan.
Interface sections for lan and wan in /etc/config/network should then look like
config interface 'lan'
option ifname 'lan0 lan1 lan2 lan3'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.1.1'
option netmask '255.255.255.0'
option delegate '0'
[...]
config interface 'wan'
option ifname 'eth0 eth2'
option proto 'dhcp'
But if and how this works with mwan3 I don’t know - you cannot have different IP-adresses for one interface…