In a small bowl, whisk together chili powder, cumin, salt and cayenne together until blended. Sprinkle the mixture evenly over the tilapia filets, coating both sides.
Heat olive oil in a medium saute pan over medium-high heat. Add filets and cook for 2-3 minutes per side until the fish is cooked through and flakes easily with a fork. Remove from heat, and break up the fish into smaller pieces with a fork.
Portion the fish evenly into the bottom of the four taco shells, then layer with lettuce, avocado, and then sprinkle with lime juice to taste. Serve immediately.