This won't hurt anything. The VCPU licensing limits the number of cores allocated to data-plane processing (how many TMM processes to run). The extra unlicensed cores are utilized by the linux kernel in user space / control-plane. In an APM deployment, the user-space APMD process is responsible for calling authentication libraries (kerberos, LDAP, radius, etc) and is usually busy processing logins, so the extra head room should help.
However there's no reason not to experiment. Why not try 4 for a few days and see what the load average is, and compare this with 8?