Chock this up to rumor, but during the Legacy of the Force panel at Celebration IV, Karen Traviss mentioned that there would be four Republic Commando novels. With Hard Contact and Triple Zero currently being available, and True Colors due in October, one can only wonder what else Ms. Traviss has up her sleeve. Only time will tell.
UPDATE: OK, I give. I must be the only one on the planet (judging by emails) that missed the fact that Book #4 is on the schedule for Aug of 08. Keep in mind that this does not mean that the book will get produced as seen here or here. Thanks to everyone who let me know of my error.
UPDATE 2: Ms. Traviss writes in to assure everyone that the book is happening. Thanks Karen!