It may look like:
=DateAdd("d", -1, DateSerial(DatePart("yyyy", finish_date), DatePart("m", DateAdd("m", 1, finish_date)), 1))
For every day in a given month it calculates the last day of the given month. So for 03/30/2014 you would get 03/31/2014.
Calculation works like: Build new date which is set to the first day of the following month of a given date. Substract 1 day. Which is the last day of the month before. As we added 1 month it is the last day of the month of the given date.
Edit
Code with finished_date = TODAY + 4 Month (finished_date + 1 so it is TODAY + 5)
=DateAdd("d", -1, DateSerial(DatePart("yyyy", DateAdd("m", 5, TODAY())), DatePart("m", DateAdd("m", 5, TODAY())), 1))