Denis Simonov, editor and contributor of Firebird Language Reference, and also the author of Firebird Developer Guide, has written the "Short Firebird Migration Guide To Firebird 4.0" (also available as PDF):

1. Manual installation of Firebird 40 on Windows                                             
2. Converting the database to a new format                                                   
3. Migrating database aliases                                                                
4. Migrating user list                                                                       
5. Configuring Trusted Authentication                                                        
6. Application-level incompatibilities                                                         

Well, it is not that short: 23 pages of essential information about Firebird 4 migration.
It covers migration from 3.0 and from 2.5, since many people still use 2.5, but plan to jump to 4.0 due to native replication and improved performance.
We are expecting Firebird 4 release very, very soon, so, if you are eager to be the first to try it, download the most recent snapshot, read this Migration Guide, and do the test migration!