Fix small mistake

pystencils/gpu/indexing.py

@@ -143,7 +143,7 @@ class BlockIndexing(AbstractIndexing):
                  maximum_block_size=(1024, 1024, 64), device_number=None):
         super(BlockIndexing, self).__init__(iteration_space, data_layout)
 
-        if self._dim < 4:
+        if self._dim > 4:
             raise NotImplementedError("This indexing scheme supports at most 4 spatial dimensions")
 
         if permute_block_size_dependent_on_layout and self._dim < 4: