I think you can specify the GROUPFORMAT option instead to get around that on your BY statement.
by year groupformat;
http://documentation.sas.com/?docsetId=lestmtsref&docsetTarget=p0yeyftk8ftuckn1o5qzy53284gz.htm&docsetVersion=9.4&locale=en#n00hu859qhvnx3n131n50id7n3gq
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