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Ok Its about 15 years since I played this but to get to the fourth pyramid you have to find the pyramid with all the inch worms and collect them together. Its on a long flat-ish bit of one of the pyramids and there are loads of inch worms around (the little yellow buggers that are harmless).
Basicly the inch worms try to follow you but usually you move to fast. So move around a bit so they all come to you then give em time to all gather round and *BANG* you have yourself a giant foot to fly on to the 4th pyramid. Bit random but then I didnt write the game.