In edit mode (TAB), delete half of the face (a). Select all the vertice (A-KEY), copy them (SHIFT+D) and mirror them (S-KEY, X-KEY). Then move the copied half-face (b). Now select all the vertice except those to join and press the Hide button. Select the vertice four by four with the B-KEY, and make face (F-KEY). Tip: to make things easier, you can turn on the Draw Faces button and rotate the 3D view while you make faces. Be sure to close all faces! (c1 and c2)