I configure Firebird-4.0.2.2816-0-Win32 service:

• Copy engine12.dll from Firebird 3.0.10 to Firebird 4.0.2\plugins
• Copy msvcr100.dll and msvcp100.dll from Firebird 3.0.10 to Firebird 4.0.2
• Add Engine12 support to firebird.conf:
  Providers = Remote,Engine13,Engine12,Loopback
• Set ServerMode to Classic or SuperClassic in firebird.conf.

I prepare 2 databases: fb30.fdb and fb40.fdb to try.

I open 3 cmd console and use isql to establish connection to fb40.fdb:

isql -u sysdba -p masterkey inet://127.0.0.1/c:/fb40.fdb

All 3 isql connections work and CPU load for firebird.exe processes is 0%. This is expected as no processing load yet.

I close all the isql connections and connect with fb30.fdb:

isql -u sysdba -p masterkey inet://127.0.0.1/c:/fb30.fdb

This time I can notice the cpu usage of firebird.exe process boost up. The more connection I open to fb30.fdb, the CPU load is higher.

This happen to Firebird 4.0 using SuperClassic and Classic mode. It doesn't happen to Super mode.