You assumed correctly, @dannjules ! The 2 ohm setting is the one.
Ah,
@MartinLY is faster than I am with the answer.
Here's a chart I made a couple of years ago that was approved by the designer of the Subway Series amp line, agedhorse. I had a similar question to yours when I bought the amp, as did another bass player friend who bought one around the same time, so I made this for both of us.
agedhorse approved it and it's a nice quick reference. If you didn't know, agedhorse is in this forum alot answering all questions as technically deep as you wanna go.
Chart:
You can see your cab combo of an 8 and a 4 puts you about 2.7 ohms, which only the 2 ohm switch setting can handle safely.
Some folk make a paper copy of the chart, some folk take a pic with their phone, to carry with 'em for convenience. Personally, I think it's suitable for framing, but that's just me LOL:
You can explore the main Subway thread here for any other questions you might have. High quality info and answers at an affordable price:
Official Mesa Boogie Subway Series Club