BOOK REVIEW : In the End, a Mercenary Foursome Gets Its Just Desserts : PRIVATE LIES <i> by Warren Adler</i> , William Morrow $19.95, 308 pages

Moved back a few centuries, the two mismatched couples in Warren Adler’s “Private Lies” would be giving each other love potions and declaring their passion in Shakespearean verse.

Unfortunately, the world has changed drastically in the last 400 years, and these four unprincipled New Yorkers dispense with the preliminaries and do their avowing in contemporary cliches.

Ken Kramer is a jaded adman, married to Maggie, a computer expert with “an Earth Mother quality.” Like many hucksters, Ken wanted to be a novelist, and if you give him the cues, he can still spout his favorite Hemingway passages. Maggie is his first wife but his second choice.

Eliot Butterfield’s inherited wealth has enabled him to devote himself to environmental issues and to hire Maggie to organize his philanthropies. Her admiration for her boss obvious. “Eliot doesn’t work, not for money,” Maggie explains to Ken. “He is an independent. As I told you, he thinks.”

Pensive Eliot is married to the glamorous Carol, who has managed to retain her ballet dancer’s body, although she’s lost her early ideals.

When Maggie waves to Eliot at the trendy Manhattan restaurant where she and Ken are dining, Eliot introduces Carol. She appears to be a clone of Ken’s college lover, with whom an affair ended when the woman decided that an aspiring ballerina needed freedom.

Within a few seconds, Ken realizes that Carol is no clone but the original, despite having reinvented her family and lopped a decade off her age. Instead of an accountant’s daughter from Long Island, she presents herself as the descendant of a French marquis.

Carol is Eliot’s second wife, and he’s never questioned either her age or her distinguished genealogy. He has, however, insisted on an ironclad prenuptial agreement, just in case Carol might someday abandon him for someone younger and less pompous.

Eliot is a well-preserved 50, but it’s always risky to marry a woman your daughter’s age. In Eliot’s world, there’s an 11th Commandment: Thou shalt not invade capital. While the original 10 are more honored in their breaching, this one is rigorously observed.

A week after this fateful restaurant encounter, Carol meets Ken to clarify her present situation and ensure his silence about their shared past. After years of struggle, she had given up ballet and settled for becoming Mrs. Eliot Butterfield. Their eyes lock and so, in short order, does everything else. It’s the 1960s all over again, only better, because they’re both more experienced.

Soon Friday afternoons in the hotel room cease to be enough, and Ken and Carol begin using dangerous words like always and forever . Wouldn’t it be marvelous if Eliot and Maggie could also fall in love? They already share so many professional interests.

Two divorces would be difficult, but a four-way, no-fault switch would be ideal.

That way, Eliot would become the adulterer, enabling Carol to avoid the financial penalty in the prenuptial agreement. Earth Mother Maggie would have Eliot to nurture, leaving Carol and Ken to fulfill the dream interrupted two decades ago.

(This is the point where the love potion would be administered, but conventions and ethics have both deteriorated.)

When Eliot proposes an African safari, Ken and Maggie eagerly accept.

The offer not only feeds directly into Ken’s Hemingway fantasies but presents unique opportunities to encourage a sexual attachment between Maggie and Eliot.

Perhaps because they are so besotted with each other, Ken and Carol have remained oblivious to the fact that Maggie and Eliot are way ahead of them with an analogous agenda of their own.

Unfortunately, Africa isn’t the Forest of Arden, and matters don’t work out quite so sweetly anymore, especially when the lovers are mercenary, hypocritical and amoral instead of romantic, winsome and virginal.

Despite their elegant outward appearances, these characters are less than the sum of their private parts. Ken and Carol and Eliot and Maggie all get their just desserts, but some will seem more just than others.