When you put the following code in a *.pro file, Qmake will exit with an error:

!contains(QT_MAJOR_VERSION, 4) {
error("you are using an old version of Qt")
}

On Linux with Qt 4.8.3 libs compiled from source, it works fine.
On windows with Qt 4.8.2 libs compiled form source (and the same MinGW compiler), it works fine.