You should also consider whether you want to store months (e.g. Feb in C4) as text.
Without knowing anything about your proposed application it is not possible to decide whether this is the best approach.
If you are using real dates in your application, you can enter a date, such as 01/Feb/2009 in a cell. Excel will recognize it as a date and then you can format the cell with "mmm" and it will show Feb.
Format with "mmmm" and the cell will show February, or
Format with "mm" and it will show 2, - no formula required.
The formula DerbyDad03 gave you works by making Excel think there is a date '1Feb' which it converts to 01 Feb 2009 and then returns the month number (2).
Try =YEAR(1&C4) and you get 2009.
So you may be better off starting with a real date in C4.