Answer:
Each of the $4\;\text{KB}$ block will contain $1$ file if all files are of $1\;\text{KB}$ size, and other $3\;\text{KB}$ space will be wasted. This is because it is not permitted to keep two files in a single block. So, it will make the wastage of $75\%$ space.
In real practice, since most fo the file systems have a mixture of small and large files altogether, hence space is used more efficiently.
Example: For a file of size $33,423\;\text{Bytes}$, $9$ Disk Blocks will be used for the storage and the efficiency will be $\frac{33,423}{34816}\times 100 = 95.999\;\%$