The K-4 visa is valid for two years with multiple entries, or until the age of 21, whichever is shorter.
Yes, you may apply for extension of stay using Form I-539, Application to Extend or Change Nonimmigrant Status, 120 days prior to the expiration of your authorized stay. Extensions will be granted in two year intervals.
Extensions for K-4 status must be filed concurrently with your K-3 parents' status extension application. In addition, your U.S. citizen parent filing for extension of your K-4 status should file Form I-130, Petition for Alien Relative, on your behalf.