dbs/db4/concurrent_access is a SAP Parameter attribute which is used to control Enable Lockless Commited Reads 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 .
Concurrent Access allows multiple processes to read committed records
without waiting for locks held by other processes updating records. In
Concurrent Access mode, the database always returns the "currently
committed" value.
For more information, see SAP Note 1804035.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.