Can you specifically explain to me why Qt 5 is better than Qt4? Are you talking about Qt service?
Qt 4 reached End-of-Life status in December 2015. This means it is no longer supported -- it will not receive any new features, bug fixes, or security fixes.
If someone finds a security vulnerability in Qt 4, that means they could attack any computer that runs Qt 4 software. The user of the software will have no protection because the flaw will never be fixed. In contrast, if such a vulnerability is found in a supported version of Qt 5, the vulnerability will be fixed urgently so that users cannot be attacked.
If you find a bug in Qt 4 that affects your project, there is not much you can do about it -- you'll need to find your own workaround to the bug or patch Qt yourself. In contrast, if you find a bug in a supported version of Qt 5, you can file a bug report and there's a good chance it will be fixed.
In any case, better have some more information about your project otherwise it's only going to be speculations.
What specific kind of information?
For starters: What is your development PC's OS? What is your deployment target's OS? Will the final product be released to customers, or is it for internal use only? How do you plan to maintain the project? Which features of Qt do you plan to use?
Also, remember to start reading http://doc.qt.io/qt-5/deployment.html