Are 4 Soft Sleeper Tickets booked together guranteed in the same compartment?

We are 4 persons and like to take K210 train on Dec/28/2017 departing Guangzhou(20:35) and arriving Hangzhou(13:38) next day. Soft Sleeper seats are available and we can book at the same time. Is it guranteed we will be in same compertment ? If so how do we know.

Thanks in advance for your answer.