Tyrosine-protein kinase that acts as cell-surface receptor for fibroblast growth factors and plays a role in the regulation of cell proliferation, differentiation and migration, and in regulation of lipid metabolism, bile acid biosynthesis, glucose uptake, vitamin D metabolism and phosphate homeostasis. Required for normal down-regulation of the expression of CYP7A1, the rate-limiting enzyme in bile acid synthesis, in response to FGF19. Phosphorylates PLCG1 and FRS2. Ligand binding leads to the activation of several signaling cascades. Activation of PLCG1 leads to the production of the cellular signaling molecules diacylglycerol and inositol 1,4,5-trisphosphate. Phosphorylation of FRS2 triggers recruitment of GRB2, GAB1, PIK3R1 and SOS1, and mediates activation of RAS, MAPK1/ERK2, MAPK3/ERK1 and the MAP kinase signaling pathway, as well as of the AKT1 signaling pathway. Promotes SRC-dependent phosphorylation of the matrix protease MMP14 and its lysosomal degradation. FGFR4 signaling is down-regulated by receptor internalization and degradation; MMP14 promotes internalization and degradation of FGFR4. Plays a role in postnatal lung development. May be involved in the development of skeletal muscle cell lineages. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Present in an inactive conformation in the absence of bound ligand. Ligand binding leads to dimerization and activation by autophosphorylation on tyrosine residues (By similarity). Monomer. Homodimer after ligand binding. Interacts with FGF1, FGF2, FGF4, FGF6, FGF8, FGF9, FGF16, FGF17, FGF18, FGF19, FGF21 and FGF23 (in vitro). Binding affinity for FGF family members is enhanced by interactions between FGFs and heparan sulfate proteoglycans. Interacts with KLB; this strongly increases the affinity for FGF19 and FGF23. Affinity for FGF19 is strongly increased by KLB and sulfated glycosaminoglycans. KLB and KL both interact with the core-glycosylated FGFR4 in the endoplasmic reticulum and promote its degradation, so that only FGFR4 with fully mature N-glycans is expressed at the cell surface. Identified in a complex with NCAM1, CDH2, PLCG1, FRS2, SRC, SHC1, GAP43 and CTTN. Interacts with MMP14 and HIP1. Interacts with STAT3 (By similarity). Internalized from the cell membrane to recycling endosomes, and from there back to the cell membrane. Isoform 1 and isoform 2 are expressed in lung and proliferating myoblasts and myotubes of primary myogenic cells (at protein level). Isoform 1 and isoform 2 are expressed in liver, muscle, spleen, heart, lung, kidney and in primary myogenic cells. Expressed in the developing gut endoderm, in myotomally derived skeletal muscle, the adrenal cortex, kidney and condensing cartilage. Isoform 1 and isoform 2 are up-regulated by estradiol during myogenic differentiation and down-regulated in fully developed myotubes. N-glycosylated. Isoform 1 and isoform 2 are glycosylated. Full maturation of the glycan chains in the Golgi is essential for high affinity interaction with FGF19 (By similarity). Ubiquitinated. Subject to proteasomal degradation when not fully glycosylated (By similarity). Autophosphorylated. Binding of FGF family members together with heparan sulfate proteoglycan or heparin promotes receptor dimerization and autophosphorylation on tyrosine residues. Autophosphorylation occurs in trans between the two FGFR molecules present in the dimer. Isoform 1 and isoform 2 are phosphorylated on tyrosine residues (By similarity). No visible phenotype. Mice display an elevated bile acid pool and elevated excretion of bile acids, due to loss of normal regulation of CYP7A1, the rate-limiting enzyme in bile acid synthesis. When on a normal diet, mice are prone to develop increased levels of white adipose tissue, hyperlipidemia, hypercholesterolemia, glucose intolerance and insulin resistance. Mice lacking both FGFR3 and FGFR4 display pronounced dwarfism, and while their lungs appear normal at birth, they are completely blocked in alveogenesis and do not form secondary septae to delimit alveoli. These mice also show elevated serum levels of 1,25-dihydroxyvitamin D3 and reduced serum phosphorus levels. Belongs to the protein kinase superfamily. Tyr protein kinase family. Fibroblast growth factor receptor subfamily.