with LM Pro 3.x, I use Brouter as the routing service when planning routes in LM, and then export the routes as GPX to my Garmin device for the actual navigation. With LM4, can I do the same?
There should be no difference so yes. External BRouter was supported in Locus Map Free so this "feature" continues to work. You may try it on your own, there is no need for any subscription to use BRouter from Google Play with Locus Map 4.