You don't expect credibility in summer's action-adventure movies, but at least "Least Weapon 4" includes one statement which is absolutely true. "We're too old for this," complains Detective Martin Riggs (Mel Gibson) to his partner Roger Murtaugh (Danny Glover). Truth to tell the two should have quit after the snappy, original first time around, one sequel tops. This time we get the usual fine action sequences and look in on the martial arts skills of a new talent. He is handsome Jet Li as a gangster who is so cool that he can stand still, arms folded, grinning, and watch his buddies try to fight off the cops, jumping into the fray only when his team is about to lose. With a few leaps, kicks and thrusts of his deadly hands, he is invincible, once even fighting off both Riggs and Murtaugh with a pole penetrating his entire body from belly to back.

With Richard Donner once again at the helm, "Lethal Weapon 4" blends a combination of good-natured humor, slapstick, violence and even some maudlin sentimentality. Partners Riggs and Murtaugh, both promoted from sergeant to captain overnight, wisecrack their way from the usual alpha to the customary omega in rounding up an ending the careers of some fun gangster and evil mobsters alike. The supporting cast have joined up once again in similar capacities: Joe Pesci performs in the role of a private investigator, just as noisome and as cuddly as he was in "Lethal Weapon 2." Rene Russo is Lorna Cole, who is Riggs' significant other, Stuart Wilson is the put-upon c.o. of the detective duo and Chris Rock joins the crew as a new breed of cop and one who is secretly married to Detective Murtaugh's daughter.

The story opens on an action sequence that has nothing to do with the principal plot, in much the style of the James Bond series. The daring duo tackling a nut case who thinks he's a video game: a guy covered from head to toe in impenetrable armor who is torching all the stores on a city block, shooting at the two main cops from an automatic weapon while Riggs and Murtaugh banter about their coming child and grandchild respectively. After dispatching the unprincipled varmint, the detectives take action against a group of smugglers, enlisting the help of their Sancho Panza, Leo Getz (Joe Pesci), Private eye. The plot is almost as impenetrable as "The X-Files: Fight the Future" but seems to involve a plan to bring Chinese illegals into the country to work as indentured servants for the bootleggers. In a design involving two rival Chinese gangs, the Triads seek to buy these slaves back from the Four Fathers using counterfeit Chinese money.

Though the detectives realize eventually that the only way to defeat these master criminals is to have them fight it out among themselves and kill one another off, they are themselves in heaps of trouble. In one situation they are left to die in a burning house because criminals still have not learned: shoot the cops first. In another, the two are involved in an underwater skirmish with the picture's karate kid.

For the women in the audience, presumably, director Donner milks some sentiment by having Murtaugh adopt a hapless family of Chinese illegals (who are ultimately granted asylum even though such asylum has rarely been granted before to folks in their situation). A terminally silly scenes is tacked on in the end, involving the usual jokes about a pregnant woman about to give birth but is "not ready" to do so.

"Lethal Weapon 4" is simply an assortment of takes involving jokes and jocks, a wholly unncessary appendage to Gibson and Glover's previous work, but one which undoubtedly will hit the number one spot in the box office given the lack of competition during the weekend of its opening.