def split(x, length):

    splitted_string = x.split(',')
    result = [substr.strip() for substr in splitted_string if len(substr.strip())==length]
    result = ', '.join(result)
    return result


df = pd.DataFrame({'a':["H33, H431, H450", "H21, H11, H521"]})

for length in [3, 4]:
    df[length] = df['a'].apply(lambda x: split(x, length))

>>> df.drop(['a'], axis=1)
          3           4
0       H33  H431, H450
1  H21, H11        H521