Multiply $4$ by $ \dfrac{r}{r+4} $ to get $ \dfrac{ 4r }{ r+4 } $.

Step 1: Write $ 4 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 4 \cdot \frac{r}{r+4} & \xlongequal{\text{Step 1}} \frac{4}{\color{red}{1}} \cdot \frac{r}{r+4} \xlongequal{\text{Step 2}} \frac{ 4 \cdot r }{ 1 \cdot \left( r+4 \right) } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 4r }{ r+4 } \end{aligned} $$

Add $ \dfrac{4r}{r+4} $ and $ \dfrac{16}{r+4} $ to get $ \dfrac{ 4r + 16 }{ \color{blue}{ r+4 }}$.

To add expressions with the same denominators, we add the numerators and write the result over the common denominator.

$$ \begin{aligned} \frac{4r}{r+4} + \frac{16}{r+4} & = \frac{4r}{\color{blue}{r+4}} + \frac{16}{\color{blue}{r+4}} =\frac{ 4r + 16 }{ \color{blue}{ r+4 }} \end{aligned} $$

Simplify $ \dfrac{4r+16}{r+4} $ to $ 4$.

Factor both the denominator and the numerator, then cancel the common factor. $\color{blue}{r+4}$.

$$ \begin{aligned} \frac{4r+16}{r+4} & =\frac{ 4 \cdot \color{blue}{ \left( r+4 \right) }}{ 1 \cdot \color{blue}{ \left( r+4 \right) }} = \\[1ex] &= \frac{4}{1} =4 \end{aligned} $$