With all the warmth of last night’s broccoli and the complexion of an avocado, Shrek has never been what one would call charismatic. “I really think he is a Tony Soprano for kids,” says “Shrek Forever After” director Mike Mitchell, not kidding. “But he’s allowed to be grumpy. ‘Cause at the same time, he’s lovable.”

True. In the grand tradition of Daffy Duck, Ralph Kramden, Homer Simpson and Stewie Griffin (“Family Guy”), Shrek has made a questionable disposition his calling card, ever since the original “Shrek” rolled out of the fairy tale swamp in 2001. Proving that a good nature doesn’t get you everywhere, “Shrek” founded a franchise that continued Friday with its fourth installment, the 3-D “Shrek Forever After,” which may or may not be the big guy’s ugly-duckling-as-swan song — Hollywood farewells, after all, are only worth the paper they’re printed on. But as it stands, Shrek — an animated movie hero made for grown-ups — is saying goodbye to his public. And his cast.

“It wasn’t until I started getting asked questions about it that it really sank in,” said Cameron Diaz, who has played the bewitched Princess Fiona in each of the movies. “My heart broke a little bit. It’s been quite a journey the last 10 years, and they’ve been a constant in my life.”

In the public’s, too. But as Mitchell says, the story “evolved” to its natural conclusion. Starring Mike Myers, Eddie Murphy and Antonio Banderas, “Shrek Forever After” finds its lime-flavored hero teetering on the precipice of a full-blown psychotic episode. After love, dragons, marriage, in-laws, epic battles, belching, farting and fatherhood, the big guy still isn’t happy. “He always wanted to be back at the swamp,” Mitchell said. “He’s finally there, surrounded by the people who love him, and he’s still not satisfied. It’s kind of a midlife crisis.”

Never mind that Shrek (Myers) isn’t supposed to be happy. In a treacherously magic deal, the devious Rumpelstiltskin (Walt Dohrn) asks Shrek to give up just one day of his past life. Shrek agrees. Rumpelstiltskin makes it the day of Shrek’s birth — hence, no Shrek. So in this parallel universe where Shrek never existed, everyone is different, the way they would have been had he never been born. Donkey is working for witches; Fiona is the flame-haired warrior chief of an insurrection, and Puss in Boots weighs 300 pounds. What it amounts to is “It’s a Wonderful Life,” transplanted from Pottersville to the kingdom of Far Far Away.

“Shrek Forever After,” as it must, reprises its trademark elements — Murphy’s antic Donkey, Banderas’ suavely self-absorbed Puss and the valiant Fiona. But each one gets a little tweak.

“She’s what she’d have been if Shrek had never been born,” Diaz said of Fiona, “so she’s kind of rough around the edges, kind of badass. She’s in all her glory, but I’ve always thought of her as a warrior, a warrior of love for her and Shrek. And it’s been a lot of fun watching her evolve, even physically — with each generation of animation they’re able to do more for each of the characters. In this one, for instance, to do her flowing hair was one of the most difficult pieces of animation they’ve had to do.”

Mitchell agreed about the hair — and as audiences will see, it’s impressive. But, he said, working with DreamWorks and its animation honcho Jeffrey Katzenberg meant “we never had any worry about not being able to do something.”

With new 3-D technology came a few new characters. Cookie, the rebels’ chef, is played by comedian Craig Robinson and continues a “Shrek” tradition of having men — Larry King and Regis Philbin, among them — play women. Or is she?

“I got conflicting reports,” Robinson said. “First, I was told I was playing a female ogre named Cookie. Then, I was told I was definitely male. I don’t know what to tell you right now.”

“Cookie is a female,” Mitchell said, laughing, admitting it’s hard to tell the male ogres from the females. “The other thing that’s interesting is that Cookie has a beard. With garlic braided in it.”

For Diaz, the end of the franchise means a little less enchantment all around. “Parents will sometimes say, ‘Oh, Cameron plays Princess Fiona, and the look on the kids’ faces will be like ‘NO! Say it’s not so!’ It’s like saying there’s no Santa Claus, or there’s no Easter Bunny. Which is a total lie, by the way. We all know there’s a Santa Claus and an Easter Bunny.”