You should put those labels in a container like a QVector<QLabel*>
instead of giving each of them a name, then access via a loop isn't a problem at all - just look them up in the vector by index.
If for some reason you have to declare those four labels independently, there's no problem with having a list of them on top of that as another class member.
In your class:
QVector<QLabel*> labels;
In your constructor:
// build the four labels
labels.append(label_1);
labels.append(label_2);
...
Iterate over them:
foreach (QLabel *l, labels) {
// whatever with l
}
for (unsigned i = 0; i < labels.size(); i++) {
// whatever with labels[i]
}