Bruce,I've always thought of these antennas as a sort of under size MF "Tee". The difference is that the antenna in the NDB case is two dimensional, having 4 wires at the top, rather than the two in a classic T. But I think the same principles apply. This Wikipedia article has a good illustration of current distribution:
en.wikipedia.org/wiki/T-antenna
If the top hat wires share equal currents, then the combined (far field) radiation from them will cancel. Of course it would require perfect symmetry in the antenna and ground system for a complete cancellation, but in reality, it's likely only a small fraction for NDB installations where wavelength >> top hat wire lengths
How much capacitance? I found this calculator for the case of a simple wire over a ground plane:
www.emisoftware.com/calculator/wire-over-ground-plane-capacitance/
I'd expect some proximity effect, i.e. 4 wires won't achieve 4x the capacitance of one wire, but it would provide a useful estimate of the contribution.
Good discussion.
Ed KO6BLM