I successfully updated our development environment with DNN 4.0.1.

See below:

Current Assembly Version: 04.00.01

Current Database Version: 04.00.00

Upgrade Status Report

00:00:00.031 - Upgrading to Version: 4.0.1
00:00:00.750 - Performing General Upgrades
00:00:01.140 - Installing Module File D:\DNN4.0\Install\Module\Announcements.zip:
00:00:01.531 - Installing Module File D:\DNN4.0\Install\Module\Announcements_3.1_Install.zip:
00:00:01.968 - Installing Module File D:\DNN4.0\Install\Module\Contacts_3.1_Install.zip:
00:00:02.203 - Installing Module File D:\DNN4.0\Install\Module\Discussions_3.1_Install.zip:
00:00:02.500 - Installing Module File D:\DNN4.0\Install\Module\Documents_3.1_Install.zip:
00:00:02.750 - Installing Module File D:\DNN4.0\Install\Module\Events_3.1_Install.zip:
00:00:03.140 - Installing Module File D:\DNN4.0\Install\Module\FAQs_3.1_Install.zip:
00:00:03.375 - Installing Module File D:\DNN4.0\Install\Module\Feedback_3.1_Install.zip:
00:00:03.546 - Installing Module File D:\DNN4.0\Install\Module\HTML_3.1_Install.zip:
00:00:03.765 - Installing Module File D:\DNN4.0\Install\Module\IFrame_3.1_Install.zip:
00:00:03.968 - Installing Module File D:\DNN4.0\Install\Module\Image_3.1_Install.zip:
00:00:04.125 - Installing Module File D:\DNN4.0\Install\Module\Links_3.1_Install.zip:
00:00:04.453 - Installing Module File D:\DNN4.0\Install\Module\NewsFeeds_3.1_Install.zip:
00:00:04.625 - Installing Module File D:\DNN4.0\Install\Module\Survey_3.1_Install.zip:
00:00:06.875 - Installing Module File D:\DNN4.0\Install\Module\UserDefinedTable_3.1_Install.zip:
00:00:08.406 - Installing Module File D:\DNN4.0\Install\Module\UsersOnline_3.1_Install.zip:
00:00:08.797 - Installing Module File D:\DNN4.0\Install\Module\XML_3.1_Install.zip:

Upgrade Complete

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Our Development Environment    

Web Server:
Dell 1550 Dual 1 Ghz processors with 2 Gb RAM
Windows 2003 Stadard Edition with Service Pack 1

Database Server:
Dell 7150 Quad Itanium processors with 12 Gb RAM
Windows 2003 Enterprise Edition with Service Pack 1 (64 bit)
Microsoft SQL Server 2005 Enterprise Edition (64 bit)

DotNetNuke:
DotNetNuke Version: 4.0.1
Data Provider: SqlDataProvider
.NET Framework: 2.0.50727.42
ASP.NET Identity: NT AUTHORITY\NETWORK SERVICE

I have confirmed that the DNN Search module errors I had experieced in DNN 4.0.0 have been fixed. As commented on the DNN 4.0 and Search Module Errors post by Cathal of the DNN Core Team: "this was caused by an error with the businesscontroller interface. It's been fixed in dnn4.01 which has been released".