You can think of this as the chance that one person shoots $4$ shots correctly in a row. By your method, the answer would be $\frac{21\cdot5}{100}$ for $5$ shooters which would clearly be over $1$ . It also wouldn’t make sense to say that getting a streak of $5$ is higher than that of getting $1$, since getting a streak of $5$ would require getting a streak of $1$. The correct way is not to add them, but to multiply them. The correct answer should be $\left(\frac{21}{100}\right)^4$ which is about $0.00194$