Histone 3 lysine 4 (H3K4) methyltransferases MLL3 and MLL4 (MLL3/4) are required for enhancer activation during cell differentiation, though the mechanism is incompletely understood. We have attempted to address this issue by generating two mouse lines: one expressing H3.3K4M, a lysine-4-to-methionine (K4M) mutation of histone H3.3 that inhibits H3K4 methylation, and the other carrying conditional double knockout of MLL3/4 enzymatic SET domain. Expression of H3.3K4M in lineage-specific precursor cells depletes H3K4 methylation and impairs adipose tissue and muscle development. Mechanistically, H3.3K4M prevents enhancer activation in adipogenesis by destabilizing MLL3/4 proteins but not other Set1-like H3K4 methyltransferases MLL1, MLL2, SET1A and SET1B. Notably, deletion of the enzymatic SET domain in lineage-specific precursor cells mimics H3.3K4M expression, destabilizes MLL3/4 proteins, and prevents adipose tissue and muscle development. Interestingly, destabilization of MLL3/4 by H3.3K4M in adipocytes does not affect adipose tissue maintenance and thermogenic function. Together, our findings indicate that expression of H3.3K4M, or deletion of the enzymatic SET domain, destabilizes enhancer H3K4 methyltransferases MLL3/4 and impairs adipose tissue and muscle development.