It's time for the moment of truth: the 4-hour wait for the starter culture to activate. You can check on it every hour or so to see if it's started producing bubbles.
But don't get your hopes up. It might take longer. Much longer. As in, 'you're-going-to-lose-all-hope-and-possibly-your-mind' long.
Just remember, the longer you wait, the more flavorful the sourdough will be. It's like that old saying: 'good things are worth the wait, unless you're dealing with a toddler, in which case, just give them a cookie.'
Proceed with caution (and a sense of desperation).