"Permanent" is a very long time.
As I said before, it depends on how the bump actually works and in the case of a future merger how the lists are combined. Will WN people get a new DOH 4 years earlier than their actual DOH? If not and a future merger used DOH, the bump would disappear relative to the next merger partner's people. Another example if WN people do get a new DOH but in a future merger the other airline's people got a 4 year bump - that negates this bump relative to the next merger carriers people.
I have no idea how the bump actually works for this merger - new DOH for WN people, keep true DOH but put list together as though WN peoples' DOH were 4 years earlier, etc. - but all that can be really said about the effect on any future merger is that the WN people would be higher up the WN/FL list than without the bump (assuming you're correct that the bump would affect placement on the WN/FL list). How that affects where they would be on a future merger list depends on a number of variables that I don't know but some of which you may know.
About all that can be said about a future merger is that it almost certainly wouldn't change the order of the WN/FL list. A WN employee above a FL employee on the combined WN/FL list would almost certainly still be above that same FL employee on a new merger list. But that guarantees nothing about how that WN employee would be ranked relative to an employee from the other carrier in another merger.
Jim.