There is no problem migrating from an ```ERL3``` to an ```ER4``` because interfaces names between these routers are the same.

After migration, we have to manually create the interface eth3 (because eth3 doesn't exist on EdgeRouter Lite) like this :

```
configure
set interfaces ethernet eth3 speed auto
set interfaces ethernet eth3 duplex auto
commit
save
```

After that, reboot EdgeRouter a couple of times and everything will work perfectly.