I have a dataframe below.

a = {'Id': ['ants', 'bees', 'cows', 'snakes', 'horses'], '2nd Attempts': [10, 12, 15, 14, 0],'3rd Attempts': [10, 10, 9, 11, 10]}
a = pd.DataFrame(a)
print (a)

I want to able add text ('-s') to anything which is equal to 4 characters. i have unsuccessfully tried the below. as it produces the error, ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

if a['Id'].str.len() == 3:a['Id'] = a['Id'].str.replace('s', '-s')
else:pass

I think you need loc, if need replace last s is necessary add $:

mask = a['Id'].str.len() == 4
a.loc[mask, 'Id'] = a.loc[mask, 'Id'].str.replace('s$', '-s')
print (a)2nd Attempts  3rd Attempts      Id
0            10            10   ant-s
1            12            10   bee-s
2            15             9   cow-s
3            14            11  snakes
4             0            10  horses

Solution with mask:

mask = a['Id'].str.len() == 4
a.Id = a.Id.mask(mask, a.Id.str.replace('s$', '-s'))
print (a)2nd Attempts  3rd Attempts      Id
0            10            10   ant-s
1            12            10   bee-s
2            15             9   cow-s
3            14            11  snakes
4             0            10  horses