Neil, the answer to your question must ultimately come from you, but here are some things to consider.
How wide is your display? If your shelf is only 12", then the 4-tube fixture will do fine. If it is wider, then the 8-tube fixture provides wider light source. Both fixtures output the same lumens per tube.
While the 8-tube fixture will measure twice the lumens of the 4-tube fixture on most light meters, that can be misleading. I won't bore you with the technical stuff, just that the 8-tube will cover a wider display and be better for a larger collection.
Either one will provide a stronger light source than the t8, thus be prepared to hang the fixture further from the plants or you risk burning the leaves. After the plants get accustomed to the strength of the light source, you can lower it (closer to leaves) if need be. I doubt for the genera you listed, that close is important.
I use both sizes for different reasons. In the end, the cost of the fixture is soon forgotten. The cost of replacing tubes becomes pretty much your only annual cost. Replacing 8 tubes is obviously twice the cost of 4.
Hope these thoughts help in making your decision. Either fixture will be significantly brighter than your t8 fixtures.
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