Objective Treatment options for relapsed/refractory multiple myeloma (RRMM) have expanded, but clinical burden remains high, particularly for heavily pretreated patients. This systematic literature review and meta-analysis was conducted to synthesize outcomes from clinical trials evaluating fourth line or higher (4L+) treatment regimens. Methodology Searches using Embase, MEDLINE, and CENTRAL were conducted (January 2012-March 2024), and publications from key conferences were hand-searched. Eligible studies were clinical trials that included the phase 2 dose of a National Comprehensive Cancer Network recommended intervention and whose population was ≥80% 4L+. Meta-analyses were conducted to synthesize outcomes by treatment class (CAR T-cell therapies [CAR T], bispecific antibodies [BsAb], and Other) using random effects models where possible. For outcomes where Kaplan-Meier curves were available, meta-analyses were conducted using survival function parameters fitted to each curve. Results The meta-analysis included 34 trials (5 CAR T, 3 BsAb, and 26 Other). All CAR T and 2 BsAb trials investigated BCMA-targeting interventions. The estimated overall response rates for CAR T, BsAb, and Other were 85.5% (95% confidence interval: 72.7, 92.9), 67.6% (62.0, 72.8), and 40.5% (33.3, 48.1), respectively. The 1-year overall survival was 84.4% (79.1, 88.4) for CAR T, 70.4% (66.6, 73.9) for BsAb, and 59.5% (55.8, 63.1) for Other. The 1-year progression-free survival was 59.0% (53.7, 64.0) for CAR T, 48.4% (44.2, 52.3) for BsAb, and 20.9% (17.9, 24.0) for Other. Infections occurred in 67.1% (59.6, 73.8) with CAR T, 69.2% (60.9, 76.5) with BsAb, and 60.2% (48.6, 70.9) with Other. For adverse events specific to the immune effector cell therapies of CAR T and BsAb, overall neurotoxicity occurred in 18.8% (10.0, 32.3) and 10.2% (7.2, 14.2), respectively, and cytokine release syndrome (CRS) in 92.3% (85.2, 96.2) and 71.4% (63.4, 78.3). Additional outcomes data will be presented. Conclusions This study offers a comprehensive and up-to-date synthesis of clinical trial outcomes for recommended interventions in 4L+ treatments for RRMM. While efficacy outcomes are improving with more recently approved interventions, there remains an unmet need for durable treatments with an improved safety profile in this population.