Complex sentence: Burnouts are most difficult to perform in four-wheel drive cars, since all four wheels are given power and 4WDs generally have better initial traction (the engine weight being directly over the drive wheels).
Split into simpler sentences: Burnouts are most difficult to perform in four-wheel drive cars due to the fact that all four wheels are given traction. Four wheel drive cars get better initial traction (the engine weight being directly over the drive wheels).