Nothing is given about #instructions or value of n.
In that we should consider $n$ = infinity.
So Speedup for n = infinity is $S$ = $\frac{T_{n}}{t_{p}}$
or we can write like, $S$ = $\frac{k.t_{n}}{t_{p}}$
Now this k tells us the number of stage or k is nothing but no. of cycles required in Non-pipelined processor, which is CPI.
$t_{n}$ is the cycle time.
So, $4$ = $\frac{CPI*(3+8+5+6+4)}{Max(3,8,5,6,4)}$
solving this eqn, we'll get CPI = 1.23