Answer is 256.
In 1992 the trout population was 4. Thus, initial population of trout is 4. In 1993 it grew to (4)², that is 4×4 = 16, in 1994 it grew (4)³, that is 4×4×4 = 64. This shows the trout has grown exponentially. Therefore, the trout population will be (4)⁴, that is 4×4×4×4 = 256.