Short answer: yes, it should be fine.
Longer answer: on my 200S there are two speaker output jacks. One is labeled 8 OHM SPEAKER. The other is labeled 4 OHM EXTERNAL SPEAKER. If you are running a single 4 ohm cabinet, connect it to the 4 ohm output.
Long answer: on Sunn amps, the speaker output jacks are switched jacks. If you plug into the 8 ohm jack, the output is taken from the 8 ohm tap on the output transformer. So, if you have one 8 ohm cabinet, that's the jack to use. If you have a second 8 ohm cabinet, plug one cabinet into the 8 ohm jack, and the other into the 4 ohm jack (two 8 ohm cabinets in parallel make for a 4 ohm total load). When you do this, the switching jacks disconnect the 8 ohm tap and connect the 4 ohm tap of the transformer. Plugging into the 4 ohm jack causes this to happen, so, if you have a single 4 ohm cabinet, connect that to the 4 ohm jack, not to the 8n ohm jack.
With tube amps, the output transformer matches the impedance so that the full power of the amp is delivered to the load. The 8 ohm tap delivers a higher voltage than the 4 ohm tap. For a given voltage, a 4 ohm load will draw more current than an 8 ohm load will, so no, a 4 ohm load will not cause the amp to work less hard. My best advice is to use the amp as designed, powering a 4 ohm load from the 4 ohm tap.
