[Source: AP]

It’s a throwaway line, but maybe a bit too meaningful, under the circumstances. “Gravity is setting in,” says Barney, Sylvester Stallone’s aging character in “Expend4bles,” when someone asks how he’s doing.

Indeed. Gravity is setting in throughout “Expend4bles,” a movie whose most enticing mystery is not the secret identity of its shadowy villain, but how you pronounce the film’s title. Are we supposed to enunciate the mid-word numeral, or is it merely visual? Is this what stands for a smart new spin on a tired franchise? Will we soon have “My Big F4t Greek Wedding”? Are these questions supposed to distract us from how stunningly mediocre the film is?

Perhaps we digress. This is, obviously, the fourth “Expendables” film, but our considered scientific opinion is that you needn’t see the first three to catch up. True, there’s no explanatory intro, but if you’ve seen earlier “Expendables” films, you’ll know there’s not much to know. These guys are the indestructible mercenaries who swoop in – literally, on Barney’s turboprop plane – to do dirty work in miserable places. The body count is head-spinningly high (this film, directed by Scott Waugh, returns to an R rating after a switch to PG-13 for the last installment). The dialogue is head-spinningly mundane. The flow of testosterone is, well, head-spinning.

Leading the pack, as ever, is Stallone’s Barney Ross and his expert knife-wielding best bud, Lee Christmas — Jason Statham, reveling in his Cockney charm and smiling more than usual. (This is not a bad thing. Statham has a nice smile. This may be the only good thing.) Also back are Dolph Lundgren’s Gunner and Randy Couture’s Toll Road.

And now, perhaps in a nod to the previously unrecognized fact that half the human race is female, we have Megan Fox as mercenary leader Gina. More on her in a bit. Also providing new blood is Andy Garcia as a prickly CIA handler, Curtis “50 Cent” Jackson as an ex-Marine and new team member, and two martial arts stars: Iko Uwais as ruthless arms dealer Rahmat, and Tony Jaa as quiet warrior Decha. Other additions: Jacob Scipio is the son of Antonio Banderas’ character from the last film, and Levy Tran is a new female teammate, adept with a whip chain.

Got all that? In a prelude scene in New Orleans, we reconnect with Barney, who now has salt-and-pepper hair, and a bad back — so bad, he enlists Christmas to help him recover his prized skeleton ring at a biker bar, which he’s lost in a thumb-wrestling contest. The thugs dispatched and the ring collected, it’s time to get back to work.

This means a trip to Libya, to “Gadhafi’s old chemical plant,” where aforementioned arms dealer Rahmat (Uwais) is securing detonators for a nuclear weapon. CIA handler Marsh (Garcia) needs the Expendables to stop him. The other thing you should know is that Barney is determined to unmask a shadowy figure codenamed Ocelot who’s maybe pulling all the strings.

Not surprisingly, the Expendables run into resistance. The body count mounts, and then something happens that will change the trajectory of the film. We can’t give it away, but let’s just say it brings Statham’s Christmas to the forefront for much of the film.