Co-odrinate geometry, deriving relationships between points

Well I've been doing okay working through my 1990 edition of Bostock and Chandler but have once more found a question I cannot fathom.

The point P(*a,b*) is equidistant from the y-axis and from the point (4,0). Find a relationship between *a* and *b.*

Now it seems to me that there will be a valid answer so long as *a *is greater than or equal to 2.

All of the other relationships so far have focused on the gradients of the lines in triangles and quadrilaterals.

Any push in the right direction would be appreciated.