Boston Public Library (BPL) trustees will vote Friday on a plan to shut the Dorchester Lower Mills and three other branches to cut a budget hole and concentrate on the "transformation" of the library system.

BPL President Amy Ryan said that despite 4,000-plus signatures from Lower Mills backers, usage has gone down there since the opening of new libraries in Mattapan and Milton.

At a meeting today, Trustee Donna DePrisco questioned why Lower Mills would be closed before branches in Uphams Corner and Egleston.

Trustee Chairman Jeffrey Rudman said he could not vote to close Uphams Corner and Egleston even if the branches are in worse physical shape. "Uphams Corner and Egleston deal with the most vulnerable people in this city. I would not close them."

He said Lower Mills patrons in most cases would be well served by the Mattapan branch. "Mattapan is a revolution," he said. "The Mattapan branch is what we should be all about."

Ryan said shutting Lower Mills and branches in Brighton, East Boston and South Boston would require 25 staff reductions, but would let the remaining branches maintain existing hours and even expand some programs. She said she would work with community centers and groups on ways to replace the shut branches with new services.