So not really a success this way, as I said this probably has a lot to do with WinNTSetup, but I wanted to report it anyway, it would have been nice if it could work.

After final reboot: 'Your current security setting do not allow this file to be downloaded'. After reboot: 'Can't load Internet Configuration Lib icfgnt.dll', 'Can't access grpconv.exe'. GUI mode not found: msxpsdrv.cat, d3dx9_26_x86.cat, d3dx9_29_x86.cat, msxpsdrv.cat. Textmode filecopy not found: iedks32.mui, ieframe.mui, w32tm.exe.

So everything mentioned below could very well be due to the special way WinNTSetup is working, but I'll just mention them here, not sure if there's anything that can be done about it. That he only parses dosnet.inf, not txtsetup.inf, so I expected some files to maybe not copy correctly. After that I added the SATA drivers again with DPbase and ran WinNTSetup to install this new 'XPSP4' source. Next thing I tested was integrating sp4 into the sp3 source. So running sp4 on a clean, installed, xpsp3 was successful.

Winntsetup alternative