The function of GSTM4 (glutathione S-transferase mu 4, Ensembl gene identifier ENSG00000168765) is as follows. Conjugation of reduced glutathione to a wide number of exogenous and endogenous hydrophobic electrophiles (PubMed:8203914, PubMed:8373352). Catalyzes the conjugation of leukotriene A4 with reduced glutathione (GSH) to form leukotriene C4 (PubMed:27791009). Can also catalyze the transfer of a glutathionyl group from glutathione (GSH) to 13(S),14(S)-epoxy-docosahexaenoic acid to form maresin conjugate in tissue regeneration 1 (MCTR1), a bioactive lipid mediator that possess potent anti-inflammatory and proresolving actions (PubMed:27791009). {ECO:0000269|PubMed:27791009, ECO:0000269|PubMed:8203914, ECO:0000269|PubMed:8373352}.