It does make sense to partition your subnet but it depends on how much disruption you're willing to tolerate and whether your apps are partition aware. What are your apps ?
Assuming you are running without partitions.conf file now, all hosts are full members of default partition. In order to separate out the 4 new hosts on it's own partition, all the existing hosts will also to be placed on their own partition so there is no communication possible between those groups of hosts due to the default partition requirement for SA communication.
Also, where does SM run ? Does it run on one of the existing hosts ? Is it a dedicated node ? Or does it run somewhere else (embedded in a switch) ?