The Single-A Potomac Nationals scored six runs in the bottom of the eighth frame to topple Frederick 6-1 in a must win Game 4 at Pfitzner Stadium in Woodbridge, Va.
The deciding game five at 7 p.m. Monday night in Frederick.
Zach Walters, Destin Hood and Jose Lozada each had a pair of RBIs for Cameron Selik, who notches the win.
Richmond dropped Harrisburg 2-1 to sweep the series 3-0. The Senators' lone run came on an Erik Komatsu double and then he scored on a Stephen King base hit.
Tanner Roark struck out 10 batters and allowed only two runs in the hard-luck loss. Chris Dominguez crushed a two-out, two-run homer for the Flying Squirrels in the sixth to get the victory.