You can set up a macro to load a project when ZBrush starts:
(1)To create the macro, copy this code into an empty text file (in Notepad):
[IButton,???,"Startup macro",
[FileNameSetNext,"ZBRUSH_ZPROJECTS\DefaultSphere.ZPR"]
[IPress,File:Open]
]
(2) Save the file to the ZBrush 4R4\ZStartup\Macros\Misc folder and call it StartupMacro.txt
(3) Now you need to edit the default zscript file. In the ZBrush 4R4\ZScripts\ folder find the DefaultZScript.txt file and open it in Notepad.
(4) Change the contents to this, and save the file:
//startup
[If,1,
[IPress,Macro:Macros:Misc:StartupMacro]
]
[pd]
Now whenever you start ZBrush the DefaultSphere project will be loaded.