We are in a similar upgrade process right now.  Tivoli server went from 3.7
to 4.2.  Six of fourteen clients are going two per week (yes, the old
clients work with the new server).

However, the only place I've installed the 4.2 client is on Informix servers
that were already at the 9.2 level.  Two reasons for this:  One, my 7.3
servers will be moving to 9.2 shortly and there was no reason to double the
downtime.  Two, the reason we went through the Tivoli update was that 9.21
(64-bit) Informix wouldn't work with 3.7 Tivoli, and we wanted to make real
sure 4.2 would work before we actually paid the bill on the upgrade.

One issue you MAY run into:  If you used any API applications to talk to
Tivoli, they will need to be recompiled and linked.  The API libraries have
changed.

As far as installing the Tivoli TDP for Informix, things got much simpler.
No more extracting object from the archive library.

Just repoint your variables (Tivoli path has changed) and go.