After a little bit of experimentation with the installation diskettes, I have found what each disk contains. I have listed the results at the bottom of this article.
As you can see, most of the information on the fourth disk, "System 7.5 Update 1.0 - 4", pertains to PowerTalk updates with the exception of the Automated AppleScript updates. So, if a user is not using PowerTalk or AppleScript on their system, the Installer is smart enough to know this, hence, it does not ask for the fourth disk.
System 7.5 Update 1.0 Installation Disk Contents
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System 7.5 Update 1.0 - 1
-------------------------
Description Version Where Installed?
----------- ------- ----------------
System 7.5 Update 1.0.2 System Folder
Finder 7.5.1 System Folder
Finder Update 1.1 Extensions Folder
7.5 Update Read Me N/A Hard Drive (Top Level)
SimpleText 7.5.1 Hard Drive (Top Level)
System 7.5 Update 1.0 - 2
-------------------------
Description Version Where Installed?
----------- ------- ----------------
Apple Guide 1.2.5 Extensions Folder
Color StyleWriter Pro 1.5.2 Extensions Folder
File Sharing Extension 7.6.1 Extensions Folder
LaserWriter 8 8.2 Extensions Folder
Printer Descriptions N/A Extensions Folder
SCSI Manager 4.3.1 Extensions Folder
Speech Manager 1.3 Extensions Folder
System 7.5 Update 1.0 - 3
-------------------------
Description Version Where Installed?
----------- ------- ----------------
Find File 1.1.1 Apple Menu Items
Find File Extension 1.1.1 Extensions Folder
Jigsaw Puzzle 1.0.1 Apple Menu Items
Stickies 1.0.1 Apple Menu Items
*Shut Down DA 1.1 Apple Menu Items
Apple Menu Options 1.0.1 Control Panels Folder
General Controls 7.5.3 Control Panels Folder
Keyboard 7.5.1 Control Panels Folder
Launcher 2.7 Control Panels Folder
MacTCP 2.0.6 Control Panels Folder
Macintosh Easy Open 1.1.1 Control Panels Folder
Monitors 7.5.1 Control Panels Folder
PC Exchange 2.0.3 Control Panels Folder
PC Setup 1.0.2 Control Panels Folder
WindowShade 1.3.1 Control Panels Folder
MathLib 2.0.1 Extensions Folder
ThreadsLib 2.1 Extensions Folder
System 7.5 Update 1.0 - 4
-------------------------
Description Version Where Installed?
----------- ------- ----------------
Mailbox Extension 1.1.1 Extensions Folder
DigiSign Utility 1.1.1 Apple Extras -> PowerTalk
. Folder
Apple Mail 1.1.2 Apple Extras -> PowerTalk
. Folder
Catalogs Extension 1.1.1 Extensions Folder
Direct Dialup 1.1.1 Extensions Folder
Synchronize Folders 7.5.1 Apple Extras ->
. AppleScript ->
. More Automated Tasks
. folder
Share a Folder 7.5.1 Apple Extras ->
. AppleScript ->
. Automated Tasks folder
Share a Folder (no Guest) 7.5.1 Apple Extras ->
. AppleScript ->
. Automated Tasks folder
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