The Enterprise, Workgroup and Express edition gone...
Now the new Informix editions are:
- Ultimate ( = Enterprise)
- Growth ( = Workgroup )
- Ultimate-C (free version)
- Innovator-C (free version)
- Developers (keep the same)

For mor information, check the links bellow...
IBM Informix ? relational, embeddable and hassle-free database
Informix technology: New Informix editions: Bargain time?
Compare the Informix Version 11 editions