Generally the outputs of an amp are wired in parallel; which means that of you use two cabinets, one in each jack, they are connected to the OT in parallel. Two 8 ohm loads in parallel would be a 4 ohm load, so you would want to use the 4 ohm tap.
This is provided that your Soldano's output jacks are in parallel. I don't know how Soldano outputs are wired, but I would certainly expect them to be parallel. You could contact the builder, or just open it up and look to be positive.