In your analysis of Fy, where does the 4kN come from? It isn't defined in the figure. But if the analysis is to be correct then it is a force in the positive y direction (upward) at point A, so let's go with that.
Why would the two be equal? There is no law that says that this must be so. The thing you showed that if you want the Y-forces to be zero, then there is still a net moment, and if you want the moment to be zero, then there is still a net force.
In other words, there is no Cy to be found where the beam is in equilibrium.