It will work in 4-tier too (exactly like how it worked in 2-tier) except the custom program will execute on the 4-tier server.
I tried in my DEV environment- 4 Tier . My custom program didnt get invoked at all.
Is there any other extra settings need to be done apart from dropping into TCRoot/bin ?
Does your program invoke any UI or requires login?
If your 4-tier server is configured to run pool manager/tcservers as a service, then there is no "login" associated with the tcserver process.
Further if no one is logged in on 4-tier server (i.e. all processes are running in background), then any program that requires login or UI will not work.
My custom program was referring to Mapped-Network-Drive which is fine with 2T. But in 4T, mapped drives are not recognized. Alternatively changing to UNC as suggested by Fegade resolved the issue.