It is true that there are two square roots of 4, which are 2 and -2.

However, often we use "square root" to mean "principal square root" which is the nonnegative value, 2. With a radical sign the value is by definition the principal square root. If we say "thesquare root" this is a pretty clear indication that we mean principal square root. Otherwise we would say "asquare root." I think it is a slight abuse of terminology but doesn't really cause communication problems.

Square root - Wikipedia, the free encyclopedia