Subtract $ \dfrac{1}{5x} $ from $ \dfrac{1}{4} $ to get $ \dfrac{ \color{purple}{ 5x-4 } }{ 20x }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 5x }$ and the second by $\color{blue}{ 4 }$.

$$ \begin{aligned} \frac{1}{4} - \frac{1}{5x} & = \frac{ 1 \cdot \color{blue}{ 5x }}{ 4 \cdot \color{blue}{ 5x }} - \frac{ 1 \cdot \color{blue}{ 4 }}{ 5x \cdot \color{blue}{ 4 }} = \\[1ex] &=\frac{ \color{purple}{ 5x } }{ 20x } - \frac{ \color{purple}{ 4 } }{ 20x }=\frac{ \color{purple}{ 5x-4 } }{ 20x } \end{aligned} $$