Answer:
Answer is
[tex]4g \: m {s}^{ - 1} [/tex]
Explanation:
We have to find the initial velocity, u.
Final velocity is v = 0. And acceleration is due to gravity.
Now,
[tex]v = u + at[/tex]
[tex]0 = u + ( - g)m {s}^{ - 2} \times 4s[/tex]
[tex]u = 4g \: m {s}^{ - 1} [/tex]
Therefore, the final answer depends on the value of g.