You need a step by step upgrade. As the Release Notes mention:
Release 9.4(2) can only be upgraded from 9.3(4) and later. Releases prior to 9.3(4) have to be upgraded to 9.3(4) first
SIP 9.3(4) can be upgraded only from Release 9.3(1) and later. This limitation applies to SIP 9.3(4) only; SCCP 9.3(4) is not affected.
The Cisco IP Phone 8941 and 8945 support a new "secure by default" feature with this release. Before phones are upgraded to Firmware Release 9.3(1), the Cisco Unified Communications Manager must have devpack 8.0.3(24049)/8.5.1(14070)/8.6.2(22030) or higher installed. The phones require this devpack. If the correct devpack is not installed first, the phones cannot successfully register to the Cisco Unified Communications Manager.
So I guess you need to
1) install latest device pack for your CUCM
2) install firmware 9.3(1)
3) install firmware 9.3(4)
4) install latest 9.4 firmware
If you have all firmware versions already installed to you can try to load one by one using the "Phone Load Name" field at the device > phone page.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.