We have plans to upgrade our AIX operating system to version 4.3. We are using Informix 7.31. Do we need any new Informix components (compilers etc.)?
You probably need to upgrade the Informix package. I'm not an AIX user, so I don't know the specifics of your upgrade, but as a general rule, it's a good idea to check with Informix and see what versions are available.
Is the system under a support contract? If so, Informix will probably cut you a deal on an upgrade of the OS/database. Check with your local sales office.