I understand that one can only bentch if eats a kezayis in 4 minutes does that have to be right away or at any point in the meal? If one does not eat a kezayis in 4 minutes and then eats something else does he make a brocha?
Eating a kzayis within the prescribed time at any point in the meal will require Birchat Hamazon. However, eating other foods [other than drink to help wash down the bread or dipping the bread] may not be exempt from their own bracha until a kzayis is eaten. So ideally a kzayis should be eaten before other foods.
Magen Avraham O:C 177:1
Igros Moshe O:C Vol. 4:41