Upgrading 32-bit SQL Server 2005 Express (SP3) Installation to SP4

PROBLEM

I am unable to upgrade a 32-bit SQL Server 2005 Express SP3 installation to SP4. When I attempt to, the installer bypasses the engine stating, "The edition of product instance MSSQLSERVER does not qualify for this update." in the associated upgrade log.

BACKGROUND

The version and edition of SQL Server I'm working with:

Microsoft SQL Server 2005 - 9.00.4035.00 (Intel X86) 
Nov 24 2008 13:01:59 
Copyright (c) 1988-2005 Microsoft Corporation
Express Edition on Windows NT 5.2 (Build 3790: Service Pack 2)

The following is a portion of my upgrade summary (...\Microsoft SQL Server\90\Setup Bootstrap\LOG\Hotfix\Summary.txt):

>>> START >>>

KB Number: KB2463332
Machine: OBFUSCATED
OS Version: Microsoft Windows Server 2003 family, Standard Edition Service Pack 2 (Build 3790)
Package Language: 1033 (ENU)
Package Platform: x86
Package SP Level: 4
Package Version: 5000
Command-line parameters specified:
Cluster Installation: No

**********************************************************************************
Prerequisites Check & Status
SQLSupport: Passed

**********************************************************************************
Products Detected                         Language  Level  Patch Level       Platform  Edition
Setup Support Files                       ENU                     9.4.5000  x86       
Database Services (MSSQLSERVER)           ENU       SP3    2005.090.4035.00  x86       EXPRESS
SQL Server Native Client                  ENU                 9.00.5000.00  x86       
Client Components                         ENU       SP3           9.3.4035  x86       EXPRESS
Microsoft SQL Server VSS Writer           ENU                 9.00.5000.00  x86       

**********************************************************************************
Products Disqualified & Reason
Product                                   Reason
Database Services (MSSQLSERVER)           The edition of product instance MSSQLSERVER does not qualify for this update.
Client Components                         The edition of product instance SQL Tools does not qualify for this update.

**********************************************************************************
Summary
     Product instances were disqualified due to product SKU mismatch
     Exit Code Returned: 11204

>>> END >>>

ACTIONS ATTEMPTED

I've tried deploying the following service pack files on this machine:

• SQLServer2005SP4-KB2463332-x86-ENU.exe
• SQLEXPR.EXE
• SQLEXPR32.EXE

NOTE: For the last two, I've called the setup utility from the commandline with the upgrade option e.g.,:

SQLEXPR32.exe /qb UPGRADE=SQL_Engine INSTANCENAME=OBFUSCATED

I'm lost. How can something so stupidly easy be so #$%&!@ complicated!

~CA