I've got a pretty good memory of this one; it was fairly powerful and not an especially bad movie, but it must have been fairly low budget and even a TV movie. At a guess, I would say it was from the 1980s, possibly earlier and almost definitely no later than 1990. It was an American film too. At least I'm pretty sure it was.
From what I remember, the story is that terrorists hijack a plane. They kill at least one of the passengers - I've got a feeling it was a woman who they shoot in a fairly blunt and shocking scene. ... The passengers rebel. The terrorists are overwhelmed and the passengers regain control of the plane. ... [T]he camera moving past the rows of seats towards the back of the plane. Then empty seats. Then the limp legs of the terrorists hanging from above.
I definitely remember watching a similar movie on TV. I don't remember all of what you mentioned, but the parts I do remember I quoted above.
Searching the IMDb turns up
Hostage Flight (1988), a made for TV movie that sounds like your film.
I do seem to remember some things that are not mentioned by you or by the IMDb, but perhaps I am conflating two movies in my head. I seem to remember there being more than just four hijackers, and I seem to remember them explicitly being Islamists. This last I recall because I remember them searching the passengers to find any Jewish people; at least one older Jewish couple tried hiding something that would give them away.