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 (By similarity).