Sarge,4H1 on Map 2 is correct. The Americans may enter anywhere along the northern edge on or west of that point.
On subsequent days of CG1 are the American's limited to entering on hexes they controlled the previous day? or can they always enter on the North edge west of 4H1? CG11 says friendly controlled hexes, so I assume the Japanese have the opportunity to control board edge hexes and limit the American entry area.