append 0's to dataframe string column if not specific length

I have a dataframe with a column object.

How do I append "0" at the end of strings which are of length 4.

Column
12356
1287
23868
5643
45634

You can use str.ljust whick takes as first argument the width that the string should be and the character to be filled with (on right side of string) if the width is less than the specified:

df['col'] = df['col'].str.ljust(width=5, fillchar='0')

print(df)

     col
0  12356
1  12870
2  23868
3  56430
4  45634

Data setup:

import pandas as pd

df = pd.DataFrame(
    {'col':'12356 1287 23868 5643 45634'.split()}
                  ,dtype=('object'))

     col
0  12356
1   1287
2  23868
3   5643
4  45634

One more method can be done using list comprehension

import pandas as pd

df = pd.DataFrame(
    {'col': '12356 1287 23868 5643 45634'.split()}
    , dtype=('object'))

df['col'] = [i + '0' if len(i) == 4 else i for i in df['col']]

output col:

     col
0  12356
1  12870
2  23868
3  56430
4  45634