Smallville could have introduced Lois Lane earlier than they did, but DC stood in the way of their original plan. In the fourth season of the Superman prequel, Erica Durance’s Lois Lane was brought on board as the cousin of Chloe Sullivan (Allison Mack). Lois served as a recurring character in season 4, before being promoted to series regular in season 5 for the remainder of the series’ ten-season run.
After being taken away by Jor-El in the Smallville season 3 finale, Clark (Tom Welling) mysteriously reappeared in a corn field in the season 4 premiere, where he was found by Lois Lane. At the time, Clark was being controlled by the Kryptonian side of his persona, who was determined to find the Stones of Power. After returning to his senses, Clark teamed with Lois and helped her save Chloe, who had previously faked her death to protect herself from Lionel Luthor (John Glover). Once Chloe was saved, Lois stayed in Smallville for a while and lived on the Kent Farm, where she often quarreled with Clark. Since Lana (Kristin Kreuk) was Clark’s primary love interest during his early years, Smallville didn’t start building toward a romance between him and Lois until they began working together as journalists at the Daily Planet in season 8.
Smallville creators Alfred Gough and Miles Millar have revealed in the past that they had initially sought to introduce Lois prior to her debut in the season 4 premiere. They wanted to start using her during Clark’s junior year in high school in season 3, but DC was opposed to this idea. That’s because when season 3 was in development, DC was already working on a Superman movie. This film eventually became Superman Returns, which served as a sequel to the films with Christopher Reeve, but in its early stages DC was thinking of having their new Superman movie be about a young Clark and Lois. DC was unwilling to let Smallville use a younger Lois since that was their plan as well [via The New York Times].
When that changed, the WB was able to negotiate with DC to have Lois in Smallville season 4. They wanted her for a full season, but DC only allowed her to appear in 13 episodes. This explains why she temporarily left Smallville to go to college in Metropolis in the first half of the season, and was absent in several other episodes.
Starting in Smallville season 5, Lois was finally made a series regular, but she remained a supporting character for a few more years, as the focus was still on Clark, Lana, Lex, and Chloe. When Lex, Lana, and Lionel all left after season 7, the show went through a major transformation with Clark transitioning from the Kent Farm to the Daily Planet. As a result, Lois’ presence on the show grew considerably. While DC’s movie plans proved to be an obstacle for Smallville using characters like Batman and Wonder Woman, everything ended up working out well with Erica Durance’s Lois, who was able to become an important and much-loved character on the show.