Only 3 of us, but can I buy four beds in one Soft sleeper cabin tickets from Chengdu to Kunming?

In China, they use real name ticket booking system now so one person just can buy one tickets on the same trip and same time. And your question is not guaranteed. Because sometimes there is another passenger who need it and there is just one vacant berth. It may be treated as the passenger misses the trip, and the conductor might sold it to the passenger who need it.