To follow up, the SNS starter was active within a couple hours. By the afternoon, brew day was cleaned up and wort cooled to 65F. SNS was in full swing so I pitched the whole jug. As Denny mentioned, there was no indication the yeast had any idea it was 4 months past expiration. The end result is just as good, if not notably better, than the recipe I normally brew. Guess I'll save a pint and use it again!