QtCored4.dll is missing from your computer

I just installed Qt on Windows and have a very simple application. When I run from Qt Creator (debug or release), everything is fine. But when I double-click on the EXE on my computer, I get the following error:

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somethin.exe - System Error
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The program can't start because QtCored4.dll is missing from your computer. Try reinstalling the program to fix this problem.
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OK
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Here is the entirety of my program:

#include <QApplication>
#include <QLabel>

int main(int argc, char* argv[])
{
QApplication a(argc, argv);
QLabel l("test");
l.show();
return a.exec();
}

What step have I missed in the installation?