The ST-120 transformer can be connected to the speaker terminal with either a 4 ohm or 8 ohm tap. I'm using the 8 ohm. I asked Bob Latino about the effects of a mismatch and he said it may sound different using the 8 ohm with 4 ohm speakers (such as less bass or some other change) but there is no mechanical or electrical harm to anything. Basically he said to try both and see which one sounds better. That makes sense and I'm planning on doing that.
However, I thought I'd post here to see if anyone would like to share their experience/views of matching up the 4 or 8 ohm taps with their own speakers and how much of a difference it made.