accessing subsampled dataframe by mask on fully sampled dataframe

Question

Here is working code

import pandas as pd

full_df = pd.DataFrame({
    "a": [1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, ]
})
sub_df = pd.DataFr