Validation Rule - Should contain only alphabets and can be upto 4 characters long

Hi, I am using MDS 2012 and want to create a business rule for data validation which says,"Should contain only alphabets and can be up to 4 characters long". How can I create such a rule in MDS? Also the Code and Name attributes are 250 characters by default. How to restrict them to 4 characters if business need them to be 4 or less character?

Any help appreciated.

Thanks, Ashish Singh