This paper is the final in a four-part series outlining treatment planning at periodontal reassessment. The first article (part 1) focused on the information that should be gathered at the reassessment appointment. Parts 2 and 3 discussed systemic and local factors that can relate to residual periodontal probing depths. Treatment can involve a range of non-surgical and surgical approaches. A variety of general, practical and local site factors can affect the choice of one option over another in choosing the most predictable treatment option. Decision-making can be challenging and this paper aims to aid this process by discussing the assessment of prognosis, factors that need to be considered in decision-making and treatment options available. A flow chart to summarise this process is presented.