From my sons:
oarport instead of airport
eye browns for eye brows
constructions (you know, the instruction sheets that help you construct something, such as a piece of furniture)
compartments for apartments (fits, if you ask me)
When asked if he was out of pants, son said no, he was in of pants
Had an ant farm with one lone ant who was named Uncle Ant. When the ant disappeared, the 4 year old boy came to me crying because the ant had "made a clean getaway"