Suppose at any instance, all 4 processes are holding 1 tape drive each as follows
Hold Need Available
P1 1 1 2
P2 1 1
P3 1 1
P4 1 1
$$\begin{array}{|c|c|c|} \hline & \text{Hold} & \text{Need} & \text{Available} \\ \hline P1 & 1 & 1 & 2 \\ \hline P2 & 1 & 1 & \\ \hline P3 & 1 & 1 & \\ \hline P4 & 1 & 1 &\\ \hline \end{array}$$
The two tape drives can be allocated to P1, which will finish execution and gives its drives back to available pool, so that the other processes will get access to tapes. Any execution sequence will lead to safe state here. The system with 4 processes ensures dead-lock free environment .