Final answer:
The Fourth Lateran Council's most important topic for laypeople was the formal establishment of the transubstantiation doctrine, which significantly affected the religious practices of Catholics.
Step-by-step explanation:
The most significant topic addressed in the Fourth Lateran Council for laypeople was likely the establishment of clear dogmatic definitions, particularly the establishment of the doctrine of transubstantiation. This doctrine elucidates the belief that the bread and wine used in the sacrament of the Eucharist become the actual body and blood of Christ during the Mass.
The council also addressed issues of Church reform and took steps to combat heresies, including measures against the Cathars and Waldensians. The Fourth Lateran Council sought to strengthen the Church's structure and religious practices in many areas, including requiring Catholics to confess their sins and take communion at least once yearly, a requirement that has had a lasting impact on Catholic life.