You can upgrade to a 4-day pass, but my understanding is that you will lose the discounted price off of the 3-day pass, and end up paying full 4-day fare.
This was told to me at the Downtown Disney Walt Disney Travel Company office when I went there & asked. However, people who have actually upgraded their tickets might have better information.
Even if you do end up paying full price for a 4-day ticket, you're still getting one of the other parks for a good discount.