Get the third and forth so you can cross the rope, under which the bridge broke. When you go up to the platform with a door on the left, right and forward. I suspect you have a monkey from the door on the left to aquire the second monkey from the cage on the podeum. You should now go right, where a monkey will aid you across the gap. In this room use the bomb bugs to destroy the plants, aiming and throwing one in. Do this to the two plants and reveal chests, this also lets you cross the gaps. To open the door on the furthest side of the room, covered by a boulder, destroy the boulder with another bomb. In this room light the pillars to raise the stairs to the cage, but watch out for the floor. In one of the other rooms is the 4th and final monkey (could be the room which has vines leading to it). After you have 4, go back to the destroyed bridge gorge, and now the monkeys will aline to let you cross. Enter half way boss.