The documentation for QPicture says the following:

Note that the default formatVersion is -1 which signifies the
current release, i.e. for Qt 4.0 a formatVersion of 7 is the same
as the default formatVersion of -1.

However, this uses a hardwired version number at the top of QPicture.cpp, "mfhdr_maj", which is currently set to 11. This is the version number of Qt 4.5. The latest version of QDataStream is in fact 12, in the 4.7 stream at least (see QDataStream.h).