The fourth Laav says the souls merge, so why do people get divorced?
In the laavan, I personally don’t think a magical transformation happens during the fourth laav where the souls are united. This is something the couple has to work towards during their marriage.
In any case, divorce is discouraged in the Sikh faith and there is no concept of it in our rehit, etc. The local Sangat (congregation) or Panj Pyaare can examine a situation and provide the appropriate guidance where such a situation is arising.
For most people these days however, laavan and Anand Kaaraj are a ritual with little real knowledge. After all, if we truly believed that Sri Guru Granth Sahib would be the centre of our lives during our married life, things would be much different in our community.