it's a relation.
in order for it to be a function there can only be 1 output for each and every input.
only one instance of multiple outputs for an input makes it a relation.
your dataset provides the following mapping:
input output
1 3
1 -3
2 4
3 4
the 2 mapping to 4 and the 3 mapping to 4 are ok for a function because there is only one output for each of the inputs.
the 1 mapping to 3 and -3 is what causes this to be a relation rather than a function because you have more than 1 output for the same input.