There is no patch installer (.MSP) released for v9.4.0, instead a full (.MSI) installer is released for Adobe Reader 9.4.0.
But the steps to update your application remain the same. A simple Help > Check for Updates will still update Reader to version 9.4.0 immaterial of the fact whether the update is a patch or a full installer.
Hope this answers your query.
I have an other solution for your query.
You can update your software from 9.3.4 to 9.4.
Use the following link to update your Acrobat reader
Once you downloaded click on install.
Hope, it helps.