dbs/db4/max_hostvars is a SAP Parameter attribute which is used to control Specifies the maximum number of hostvars information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
Specifies the maximum number of host variables for each SQL statement.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.