SQL DataType - How to store a year?

If you need to store a year in the database, you would either want to use an Integer datatype (if you are dead set on only storing the year) or a DateTime datatype (which would involve storing a date that basically is 1/1/1990 00:00:00 in format).

regular 4 byte INT is way too big, is a waste of space!

You don't say what database you're using, so I can't recommend a specific datatype. Everyone is saying "use integer", but most databases store integers as 4 bytes, which is way more than you need. You should use a two byte integer (smallint on SQL Server), which will conserve space.