Why was the cast different?

In Season 4's first episode, some of the changes were explained, some were not (it's been years since I've seen this, so some essentially unimportant details have escaped me). String goes to talk to Archangel about his Vietnam MIA brother Saint John (what brought this topic up is one of those aforementioned "essentially unimportant details"), and finds Jason Locke instead (the stock shot of the building is now captioned "Company HQ" instead of "F.I.R.M. HQ"), who tells Hawke that Michael has been transferred to the Far East (he got into trouble that he couldn't talk his way out of, for which he received that transfer [which is obviously a demotion], Zeus & the Committee were disbanded/sent into retirement, and "the Company" [a nickname for the C.I.A., which "the FIRM" was a play on to begin with] absorbed F.I.R.M. operations; note also that String sports a wedding band while there is no mention whatsoever of Caitlin, suggesting that THEY are married and she's living at the cabin, away from the dangers of both Airwolf missions and movie stunt work, out of concern for Hawke's "curse" upon those he loves--all fan speculation, and yes Vincent was wed in the interim, the REAL explanation for that ring). The exploding helicopter that kills Dom (no suggestion it's anything but a legitimate accident) also severely injures String, who is hospitalized, in critical condition. Locke recruits Major Mike Rivers to locate Airwolf, which he does by examining satellite photos and finding the long-term effects of the Lady's propellor wash on the desert sands around the Lair (ingenious!). However, somehow Dom's niece Jo--unheard of prior to this episode--is there ahead of them (I am certain that this was not explained, she was simply already there when the two men entered), claiming the Lady as hers. BTW, the place has been significantly upgraded since we last saw it in Season 3; there is a sizable computer console from which an Airwolf mission can be continuously monitored and the flight crew kept in constant communication with. Locke wants Airwolf because he knows that Saint John has been working under deep cover for some branch of U.S. intelligence for many years--obviously, if our spy agencies did not keep secrets from each other, they could have had her back at the outset!--and his current mission (in some tropical country, but at this late date I don't remember if it was Latin American, African or Asian/Pacific) has gone bad and he needs to be rescued. They do so, and Saint John goes to his still hospitalized brother, who has regained consciousness. "I don't want to die in this place," String tells him. The other Hawke picks him up and carries him out. After a commercial break (in the original US telecast, anyway), we see The Lady landing in the Lair, where Jason, Jo and Mike are waiting. SJ climbs out and thanks them for allowing him to use Airwolf for a special task (burying String's body by the cabin?). Locke suggests that Airwolf can do more good if they keep her in the Lair than if they turn her over to the government as a whole. The other three agree.

BTW, gladbecker's implication that Mike Rivers was around from Season 2 is undoubtedly unintended, and he's just trying to say that of the Season Four cast, Mike replaced Cait, with which I disagree. Despite the "blood" relationship between Dom and Jo (and her assuming the reins of Santini Air), I'd peg her as the Cait counterpart, and military man Major Rivers in for WWII vet Dom.