Qt 5.1.1 Library
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Qt does not maintain binary or source compatibility across major versions. Code compiled for Qt4 will not run on Qt5 libraries, and vice versa.
There is binary and source compatibility promise inside a major version, though, so you can compile stuff using - for example - Qt 5.0.1 and then run it on Qt 5.1.
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In addition to sierdzio's comments.
You can recompile most of your application with Qt4.8. However, you have to be aware that there is also no complete code compatibility between Qt4.8 and Qt5. A few adaptions are required.
The upgrade compiling application source developed with Qt4 to Qt5 was quite easy in my case. However, I would be careful the other way around. -
You can also look at what this destinuado Qt.5.X.
Ex: I know QFTP, QHTTP been discontinued and replaced by another library, but they still are available for download, with the intention to still maintain backward compatibility.
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But you have great chances of your program done in Qt4.8.X run well on Qt5.x
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See http://qt-project.org/doc/qt-5.1/qtdoc/portingguide.html and its links for a list of incompatible changes between Qt 4.8 and Qt 5
