$\underline{\textbf{Answer:}\Rightarrow 3.2}$
$\underline{\textbf{Explanation:}\Rightarrow}$
$\color{blue}{\underline{\textbf{For Non-Pipelined System:}\Rightarrow}}$
Given frequency $\mathbf{ = 2.5 GHz}$
$\therefore 1\; \text{Cycle time} = \mathbf{\dfrac{1}{2.5Gs} }= \dfrac{1}{2.5}\;\text{nano seconds}$
$\therefore \text{Total Time} = 4\times \frac{1}{2.5} \text{nano seconds} =\dfrac{4}{2.5}\;\text{nano seconds}$
$\color{blue}{\underline{\textbf{For Pipelined System:}\Rightarrow}}$
$\textbf{Average CPI} = 1\;\;[\because \text{It is given that pipeline has no stalls.}]$
This also proves the fact that pipeline is also $\color{magenta}{\text{independent of the number of phases}}$, [so there would be $\color{red}{\text{no change}}$ if the question was asked for $\mathbf 6$ stages also.]
Similarly, as above:
$\mathbf{Frequency = 2\;GHz}$
$\therefore\;1\text{ cycle time} =\dfrac{1}{2} \;\text{nano seconds}$
$\therefore \textbf{Speed-up} = \dfrac{\text{Time without pipelining}}{\text{Time with pipelining}} = \dfrac{\dfrac{4}{2.5}}{\dfrac{1}{2}} = 3.2$
$\therefore\;\mathbf{3.2}$ is the correct answer.