Hi, sorry for the late response, for some reason I didn't get a notification about this Question.
The key when setting up the bc file is that you are setting the direction to the face that flow will come into or out of the domain. You are not specifying the direction of flow. ie At pre stage you are not setting if it's an inflow or outflow, only where flow can either come enter of leave the domain. (when running you can then have flow in and out of the domain through this face because you can have -ve inflows and outflows).
So your bc.dat file should be
1 flow_xp Inflow
4 3 :
! - - -
2 flow_xm Outflow
2 2 :