Mouse events in PyQt5 and PyQt4 - python-2.7

Question

I used to have an application in PyQt5, where user could move QGroupBox widget, by pressing mouse button and simply move it. In PyQt5 it works perfectly, but I had to refuse PyQt5 for some reasons and use PyQt4 instead of it. But now I can move my QgroupBox widget only after double click. I use the same piece of code, but in PyQt5 it worked after one click and now only after second one (after first click the whole application's window starts to move). What should I add/edit in order to reach the same result like in PyQt5?
from PyQt4.QtGui import *
from PyQt4.QtCore import *
class WidgetPattern(QGroupBox):
def __init__(self, name, parent=None):
super(WidgetPattern, self).__init__(parent=parent)
self.layout = QVBoxLayout(self)
self._offset = 0
self.widgetLength = 280
self.name = QLabel(parent=self)
self.name.setText(name)
self.name.adjustSize()
self.name.move(self.widgetLength / 2 - self.name.width() / 2, 5)
self.close_button = QToolButton(parent=self)
self.close_button.resize(24, 24)
self.close_button.setAutoRaise(True)
self.close_button.move(self.widgetLength - self.close_button.width(), 0)
self.close_button.setIcon(QIcon.fromTheme("application-exit"))
self.setFixedSize(280, 60)
def mousePressEvent(self, event):
self._offset = event.pos()
def mouseMoveEvent(self, event):
self.move(
QGroupBox.mapToParent(
self,
event.pos() -
self._offset))
class ConcertGUI(QWidget):
def __init__(self, parent=None):
super(ConcertGUI, self).__init__(parent)
widget=WidgetPattern("test")
self._main_layout = QVBoxLayout()
self._main_layout.addWidget(widget, 0)
self.setLayout(self._main_layout)
self.resize(1024, 500)
if __name__ == '__main__':
import sys
app = QApplication(sys.argv)
gui = ConcertGUI()
gui.show()
sys.exit(app.exec_())