df = pd.DataFrame({'Q1_Seen': ['Q1a', None, None, None], 'Q2_Seen': [None,
    'Q2a', None, 'Q2c'], 'Q3_Seen': [None, None, 'Q3d', None], 'Q4_Seen': [
    None, None, None, None]})
df
df['Q_Seen'] = df.max(axis=1)
Merge multiple pandas columns into new column