How to fill missing values by looking at another row with same value in one column(or more)?
Sorted and did a forward-fill NaN

import pandas as pd, numpy as np
data = np.array([[1,2,3,'L1'],[4,5,6,'L2'],[7,8,9,'L3'],[4,8,np.nan,np.nan],[2,3,4,5],[7,9,np.nan,np.nan]],dtype='object')
df = pd.DataFrame(data,columns=['A','B','C','D'])

df.sort_values(by='A',inplace=True)
df.fillna(method='ffill')