Dina Lohan -- Allegedly Ducks $4,000 Bill ... at CHARITY EVENT

Dina Lohan better hang on to the $50k she scored by screaming at Michael Lohan on a talk show last week ... because she's about to get sued for stiffing a charity ... TMZ has learned.

Lindsay Lohan's mom hobnobbed it with some rich folk at a Ferrari charity event last year in East Hampton. The shindig was held at Andrra Waterside Restaurant and Ferrari owners and nearby yachters were out in force.

Edmond Chakmakian, the lawyer for the restaurant tells TMZ ... Dina and her brother showed up and went to town, drinking up a storm and buying drinks for lots of people at the event. By the end of the night, Chakmakian says Dina ran up a $2,500 tab. Dina also pledged $1,500 to The Clamshell Foundation, which funds local charities that help underprivileged kids.

Chakmakian says Dina left without paying a cent, and he has sent her multiple emails demanding payment ... but got no response.

So Chakmakian says he's filing a lawsuit Monday against Dina on behalf of the restaurant for the $4K she didn't pay.

Chakmakian says, "She feels very privileged and entitled because of who her daughter is. She acted like she was doing them a favor by showing up."

Sounds about right.