Four layer structure in Momentum

Hello all, first post, hope it's an easy one to fix!

I am trying to model a via thru a 4 layer PCB in Momentum. I have port1, a trace on top layer, a via to bottom, then a trace to port 2. Simple.

When I have a 3 layer construction (slot type ground, with a circular hole to clear the via) all is fine and dandy, but I'm not sure where to go from here to model TWO inner grounds to experiment with the optimum position of ground-vias near the via. I'm assuming that Momentum will treat these as not connected, and I'm not sure this is valid....If I simply put in another slot ground layer and try to link both internal grounds with a pad defined between the two grounds I get a no converge, although this looks like the correct structure in the 3D viewer.

Am I not doing what I think I'm doing by defining the pad between grounds? Is this just not possible because of the way that Momentum treats its slot-type grounds?

I should say that this is inside Genesys, not ADS, but I'm not sure that should make a difference...

Any suggestions welcome, thanks!

I suggest you set the model in HFSS or CST if possible.