Maybe, it would be nice. I could certainly use another of his nifty Plasma Rifles.

I think we'll hear them being referenced at the very least. Maybe you overhear some synths or commonwealth guys talking about Harkness and his daring escape.

If we hear anything, it'll have to be very little or at least contradictory reports depending on who we hear from, otherwise saving harkness or getting him killed in FO3 will no longer be a player choice. In the same way we shouldnt hear definitively wheher project purity worked or not.

Well i guess you are only partialy correct here, game developers usualy do decide on whats cannon for sequels at least for major decisions unless there is a save import aviable that lets you carry your choices trough directly(which there isnt) so while minor choices from previus games cud be left unexplained and only hinted about i do expect big ones to have a cannon ending if they impact the story of FO4 in any way

BGS leaves things vague all the time, they never made anything official canon afaik.