I am using mwan3 and I have played a bit and helped troubleshoot pbr.
However rules and routes are the easiest in your case.
config rule
option in 'lan0'
option lookup '100'
config rule
option in 'lan1'
option lookup '101'
config rule
option in 'lan2'
option lookup '102'
config rule
option in 'lan3'
option lookup '103'
config route
option interface 'wan0'
option target '0.0.0.0'
option netmask '0.0.0.0'
option gateway '10.30.0.253'
option table '100'
config route
option interface 'wan1'
option target '0.0.0.0'
option netmask '0.0.0.0'
option gateway '10.30.1.253'
option table '101'
config route
option interface 'wan2'
option target '0.0.0.0'
option netmask '0.0.0.0'
option gateway '10.30.2.253'
option table '102'
config route
option interface 'wan3'
option target '0.0.0.0'
option netmask '0.0.0.0'
option gateway '10.30.3.253'
option table '103'
If the interface names and IPs are correct, copy paste it in config/network and restart network.