"Defiant Reclaimer, he who will shatter the land and break the sky apart..."
I have previously wondered if they catalogued themselves asked that question about a month ago, but remember we know Forerunners survived the Halo array from shield worlds and other locations - see the Iris campaign and terminal 7 of H3. Personally i believe that the Forerunner race left the Orion arm of the galaxy and moved onto the 'great journey' maybe to a distant part of the Milky Way, or another Galaxy completely. Yet some Forerunners certainly could stay behind...
P.s Halo 4 official blurb states; "ancient evil". Although a returning evil could be a "new threat" it is not "retarded" at this time to assume that the Forerunners could play this role, or the Precursors or something yet undiscovered...