Serine/threonine kinase that plays a role in the response to environmental stress and cytokines such as TNF. Appears to act upstream of the JUN N-terminal pathway (PubMed:9890973). Activator of the Hippo signaling pathway which plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. MAP4Ks act in parallel to and are partially redundant with STK3/MST2 and STK4/MST2 in the phosphorylation and activation of LATS1/2, and establish MAP4Ks as components of the expanded Hippo pathway (PubMed:26437443). Phosphorylates SMAD1 on Thr-322 (PubMed:21690388). {ECO:0000269|PubMed:21690388, ECO:0000269|PubMed:26437443, ECO:0000269|PubMed:9890973}. This is the function of Ensembl gene identifier ENSG00000071054 (MAP4K4, mitogen-activated protein kinase kinase kinase kinase 4).