A disk block has $1024$ bytes, and both a pointer and a search key are of $4$ bytes each. Now, consider that we build a $B^{+}-$tree index with a disk block to store every tree node. Assume that the index tree has a height of $4.$ Estimate the number of unique search key values that can be represented by the index tree. Precisely, you shall give the number of unque search values that can be represented by the index tree at least, and the number of unque search values that can be represented by the index tree at most.