Extact year from Varchar date

First things first: never store dates as string values - it makes live a lot, lot more complex than it needs to be, because different systems use different string representations of dates:

MM/dd/yyyy
MM/dd/yy
dd/MM/yyyy
dd/MM/yy
yyyy-MM-dd
yyMMdd
dd MMM yyyy

Among others. If you store date values as DateTime columns, then your database is always using a valid value that is a genuine date, rather than a "might be a date, might not, might be '34th Feb 2213', might be 'the date'". This complicates your software to a stupid degree!

But...if you can't get round it:

SELECT SUBSTRING(MyDateColumnAsAString, 7, 4) FROM MyTable

But don't be surprised when it crashes in production!

Another way to accede to your request is:

SELECT RIGHT(thedateinstringcolumnname, 4) from tablename

But do take note of advice from OriginalGriff and CPallini.

All above solutions are good.
I would suggest to use CONVERT[^] with YEAR[^] function:

SET DATEFORMAT dmy;
SELECT YEAR(CONVERT(DATETIME,'12/02/2013')) AS MyYear