Как сделать перетаскивание QLabel?

То что я вам покажу - это лишь одно из возможных направлений для решения вашей задачи.

from PyQt5 import QtWidgets, QtGui, QtCore
from PyQt5.Qt import *


# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv 
class DraggableLabel(QLabel):                   
    def mousePressEvent(self, event):
        if event.button() == Qt.LeftButton:
            self.drag_start_position = event.pos()

    def mouseMoveEvent(self, event):
        if not (event.buttons() & Qt.LeftButton):
            return
        if (event.pos() - self.drag_start_position).manhattanLength() < QApplication.startDragDistance():
            return
        drag     = QDrag(self)
        mimedata = QMimeData()
        mimedata.setText(self.text())
        drag.setMimeData(mimedata)
        pixmap  = QPixmap(self.size())
        painter = QPainter(pixmap)
        painter.drawPixmap(self.rect(), self.grab())
        painter.end()
        drag.setPixmap(pixmap)
        drag.setHotSpot(event.pos())
        drag.exec_(Qt.CopyAction | Qt.MoveAction)


class DropLabel(QLabel):
    signal = pyqtSignal()
    
    def __init__(self, *args, **kwargs):
        QLabel.__init__(self, *args, **kwargs)
        self.setAcceptDrops(True)

    def dragEnterEvent(self, event):
        if event.mimeData().hasText():
            event.acceptProposedAction()

    def dropEvent(self, event):
        pos  = event.pos()
        text = event.mimeData().text()
        # надо перевернуть действие
        if '-' in text:
            text = text.replace('-', '+')
        # ...    
        
        self.setText(text)
        event.acceptProposedAction()
        
        self.signal.emit()
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 


class Ui_Tester(object):
    def setupUi(self, Tester):
        Tester.setObjectName("Tester")
        Tester.resize(462, 553)
        self.centralwidget = QtWidgets.QWidget(Tester)
        self.centralwidget.setObjectName("centralwidget")
        
        self.label_2 = QtWidgets.QLabel(self.centralwidget)
        self.label_2.setGeometry(QtCore.QRect(280, 180, 15, 36))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label_2.setFont(font)
        self.label_2.setAlignment(QtCore.Qt.AlignCenter)
        self.label_2.setObjectName("label_2")
        
        self.label_x = QtWidgets.QLabel(self.centralwidget)
        self.label_x.setGeometry(QtCore.QRect(160, 180, 15, 36))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label_x.setFont(font)
        self.label_x.setAlignment(QtCore.Qt.AlignCenter)
        self.label_x.setObjectName("label_x")
        
        '''
        self.label_equals = QtWidgets.QLabel(self.centralwidget)
        self.label_equals.setGeometry(QtCore.QRect(240, 180, 19, 36))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label_equals.setFont(font)
        self.label_equals.setAlignment(QtCore.Qt.AlignCenter)
        self.label_equals.setObjectName("label_equals")
        '''
        
#        self.label_4 = QtWidgets.QLabel(self.centralwidget)
        self.label_4 = DraggableLabel(self.centralwidget)                           # !!!  
        
        self.label_4.setGeometry(QtCore.QRect(190, 180, 39, 36))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label_4.setFont(font)
        self.label_4.setAlignment(QtCore.Qt.AlignCenter)
        self.label_4.setObjectName("label_4")
        
        self.label_result = QtWidgets.QLabel(self.centralwidget)
        self.label_result.setGeometry(QtCore.QRect(150, 250, 191, 51))
        font = QtGui.QFont()
        font.setPointSize(18)
        self.label_result.setFont(font)
        self.label_result.setStyleSheet("background-color: rgb(255, 255, 255);\n"
"border-radius: 10px;\n"
"border: 2px solid rgb(29, 255, 29);")
#"border: 2px solid rgb(29, 277, 29);")
        self.label_result.setObjectName("label_result")

        Tester.setCentralWidget(self.centralwidget)

        self.retranslateUi(Tester)
        QtCore.QMetaObject.connectSlotsByName(Tester)

    def retranslateUi(self, Tester):
        _translate = QtCore.QCoreApplication.translate
        Tester.setWindowTitle(_translate("Tester", "Tester"))
        self.label_2.setText(_translate("Tester", "2"))
        self.label_x.setText(_translate("Tester", "x"))
#        self.label_equals.setText(_translate("Tester", "="))
        self.label_4.setText(_translate("Tester", "- 4"))
        self.label_result.setText(_translate("Tester", "итог: "))


class MainWindow(QMainWindow, Ui_Tester):
    def __init__(self):
        super().__init__()
        
        self.setupUi(self)
        
        self.frame_left = QFrame(self.centralwidget)
        self.frame_left.setObjectName("frame_left")
        hbox_left = QHBoxLayout(self.frame_left)
        hbox_left.setObjectName("hbox_left")
        hbox_left.addWidget(self.label_x)
        hbox_left.addWidget(self.label_4)
        
        self.label_ = DropLabel(self.centralwidget)                                 # !!! +++
        self.label_.signal.connect(self.label_hide)
        self.label_.setObjectName("label_")
        self.label_.setAlignment(QtCore.Qt.AlignCenter)
        self.label_.setObjectName("label_")
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label_.setFont(font)
        
        self.frame_right = QFrame(self.centralwidget)
        self.frame_right.setObjectName("frame_right")
        hbox_right = QHBoxLayout(self.frame_right)
        hbox_right.setObjectName("hbox_right")
        hbox_right.addWidget(self.label_2)
        hbox_right.addWidget(self.label_)
        
        self.pushButton = QtWidgets.QPushButton('=')                                # !!! +++
        self.pushButton.setObjectName("pushButton")
        self.pushButton.setMinimumSize(QtCore.QSize(25, 25))
        self.pushButton.setMaximumSize(QtCore.QSize(25, 25))
        self.pushButton.clicked.connect(self._result)
    
        layout = QGridLayout(self.centralwidget)
        layout.setObjectName("layout")
        layout.addWidget(self.frame_left, 1, 1)
        layout.addWidget(self.pushButton, 1, 2)
        layout.addWidget(self.frame_right, 1, 3)
        layout.addWidget(self.label_result, 2, 1, 1, 3)
        layout.setRowStretch(0, 1)
        layout.setRowStretch(3, 1)    

    def label_hide(self):
        self.label_4.hide()

    def _result(self): 
        s_1 = self.label_2.text()
        s_2 = self.label_.text()
        if not s_2:
            msg = QtWidgets.QMessageBox.warning(self, 'ВНИМАНИЕ !', 'Выполните перетаскивание.')
            return
        self.label_result.setText(f'итог: x = {int(s_1) + int(s_2.replace(" ", "") )}')
        

StyleSheet = '''
#label_x {
    background-color: rgba(235, 135, 135, 122);
}
#label_2, #label_4, #label_ {
    background-color: rgba(135, 235, 135, 122);
}            
'''


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
                 
    app.setStyleSheet(StyleSheet)
    
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())