Based on wxWidgets Stable Release: 3.0.5 (April 27th, 2020)
https://www.wxwidgets.org/downloads/
https://github.com/wxWidgets/wxWidgets/releases/download/v3.0.5/wxWidgets-3.0.5.zip

Modifications to compile it for WINVER=0x0400:


***** Added in .\src\msw\window.cpp (line 3526): *****
// Trying to make compatible with Win 9x
#ifndef WM_UNINITMENUPOPUP
    #define WM_UNINITMENUPOPUP 0x0125
#endif
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***** Modified .\src\msw\checklst.cpp (line 161): *****
#if (WINVER >= 0x0500) // Disable in builds for old Win9x versions
    // checkmarks should not be mirrored in RTL layout
    DWORD oldLayout = impl->GetLayoutDirection() == wxLayout_RightToLeft ? LAYOUT_RTL : 0;
    if ( oldLayout & LAYOUT_RTL )
        ::SetLayout(hdc, oldLayout | LAYOUT_BITMAPORIENTATIONPRESERVED);
    wxDrawStateBitmap(hdc, hBmpCheck, x, y, uState);
    if ( oldLayout & LAYOUT_RTL )
        ::SetLayout(hdc, oldLayout);
#endif
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***** Modified .\src\msw\utils.cpp (after line 1170): *****
***** added (WINVER >= 0x0500) check for OSVERSIONINFOEX wxGetWindowsVersionInfo()
***** code to be used only for newer Windows versions builds
***** For 9x versions used code from 3.0.0 ***** https://github.com/wxWidgets/wxWidgets/blob/v3.0.0/src/msw/utils.cpp
*******************************************************


***** Modified .\src\msw\toplevel.cpp (line 1545): *****
#if (WINVER >= 0x0500) // Disable message in builds for old Win9x versions (doesn't fix it but hides the error...)
			wxASSERT_MSG( m_menuDepth > 0, wxS("No open menus?") );
#endif
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