OATH — 4 letters
Trace OATH from row 1, column 2. Move left to row 1, column 1 for A, then turn down to row 2, column 1 for T and continue down to row 3, column 1 for H. The letters read O → A → T → H. This route uses three rows and two columns, with one turn between its leftward opening move and its downward finish.
For a careful board check, the complete sequence is: 1 at row 1, column 2; 2 at row 1, column 1; 3 at row 2, column 1; 4 at row 3, column 1. It starts with 3 open orthogonal neighbors and ends at a cell with 2 open orthogonal neighbors. All four visited cells are on the board edge, including the corner at row 1, column 1, which makes this a useful route to settle before considering the interior letters.