$x^{2}-5 x+4<0$

Step 1 ：Re-arrange the inequality: $x^2 - 5x + 4 < 0$

Step 2 ：Factor the left-hand quadratic: $(x - 4)(x - 1) < 0$

Step 3 ：Determine the intervals where the inequality holds: $x < 1$ or $x > 4$

Step 4 ：Express the answer in interval notation: \(\boxed{x \in (-\infty, 1) \cup (4, \infty)}\)