<p>I create a GUID (as a string) and get the hash of it. Can I consider this hash to be unique?</p>

Not as reliably unique as the GUID itself, no.
Just to expand, you are reducing your uniqueness by a factor of 4, going from 16 bytes to 4 bytes of possible combinations.
As pointed out in the comments the hash size will make a difference. The 4 byte thing was an assumption, horrible at best I know, that it may be used in .NET, where the default hash size is 4 bytes (int). So you can replace what I said above with whatever byte size your hash may be.