RNA-binding protein that function as a pre-mRNA splicing factor. Plays a critical role in both constitutive and enhancer-dependent splicing by mediating protein-protein interactions and protein-RNA interactions required for accurate 3'-splice site selection. It can functionally substitute for U2AF1 in constitutive splicing and enhancer-dependent splicing. Acts by enhancing the binding of U2AF2 to weak pyrimidine tracts. Also participates in the regulation of alternative pre-mRNA splicing. Activates exon 5 skipping of PTPRC during T-cell activation; an event reversed by GFI1. Binds to RNA at the AG dinucleotide at the 3'-splice site. Shows a preference for AGC or AGA. Alternative splicing of U2AF1L4 may play a role in connecting the circadian rhythm to changing external cues: may provide a circadian buffering system in central and periphery clocks that allows synchronized adaption to clock-resetting stimuli in order to prevent potentially pathogenic desynchronization