Pandas: how to change only one column which is in a series contain same column name

Question

a dataframe such as:

                  WEEK_BEG_DT  ...             WEEK_BEG_DT      OHTER
0                 2013-09-29  ...              2013-09-29       a
1                 2013-06-23  ...              2013-09-29       b
2                 2012-04-28  ...              2013-09-29       c
3                 2013-01-06  ...              2013-09-29       d
4                 2013-06-16  ...              2013-09-29       e

I only want to format the last column 'WEEK_BEG_DT' to be "%Y".

what I expected:

                  WEEK_BEG_DT  ...             WEEK_BEG_DT       OHTER
0                 2013-09-29  ...              2013              a
1                 2013-06-23  ...              2013              b
2                 2012-04-28  ...              2013              c
3                 2013-01-06  ...              2013              d
4                 2013-06-16  ...              2013              e