I tried lesson 4 with the code as done in the video (double, triple, quadruple checked the syntax), and couldn't get it to compile with Qt 5.4.

BUT I did get this to work... what's the difference between that version and this one? Are there more differences as I move into later lessons?

//(I didn't delete mainwindow.cpp)//

#include "mainwindow.h"
#include <QApplication>
#include <QLabel>

int main(int argc, char *argv[])
QApplication a(argc, argv);
QLabel *label = new QLabel("text!");

return a.exec();