Why did Tiffani-Amber Thiessen and Elizabeth Berkley leave saved by the bell midway through the fourth season?

Tiffani-Amber Thiessen and Elizabeth Berkley left "Saved by the Bell" midway through the fourth season as they sought to pursue other opportunities in their acting careers. Thiessen, who played Kelly Kapowski, and Berkley, who portrayed Jessie Spano, both felt it was time to move on from the show, which had been a significant part of their early careers. Their departures marked a shift in the series, leading to the introduction of new characters to fill the void.