Transcriptional activator that may play a role in the unfolded protein response of the testis. Proposed to be involved in spermiogenesis. May be involved in regulating the maturation of sperm head nuclei. Alternatively proposed to be a paternally delivered transcription factor that may function in early zygotic gene activation. Increases the binding of CREM isoform Tau with CRE. The CREM isoform Tau-CREB3L4 heterodimer functions through CRE but not through UPRE and may recruit HIRA to CRE to regulate histone exchange. Binds DNA as a dimer (By similarity). Forms a heterodimer with CREM isoform Tau. According to PubMed:16925989 is not observed within nuclei of haploid spermatids at any spermiogenic stage. Under ER stress the cleaved N-terminal cytoplasmic domain translocates into the nucleus. Predominantly expressed at high levels in testis with isoform 2 being the predominant isoform. Specifically expressed in postmeiotic spermatids and accumulates in the mid/late stage (at protein level). Ubiquitously expressed at low levels. Expression is very low in the prepubertal testis (2-5-, 8- and 17-day-old) but becomes abruptly induced in the postpubertal testis (29-day-old), after which expression increased (35- and 60-day-old). Controlled by regulated intramembrane proteolysis (RIP). Following ER stress a fragment containing the cytoplasmic transcription factor domain is released by proteolysis. The cleavage seems to be performed sequentially by site-1 and site-2 proteases (PS1 and PS2). PS1 cleavage may be suppressed by a determinant in the C-terminal region. Creb3l4 null-deficient mice show a reduced spermatogenesis but are fertile. Minor. Major. Belongs to the bZIP family. ATF subfamily. According to PubMed:15938716 binds to the UPR element (UPRE) but not to CRE or the NF-kappa-B site. Preferentially binds DNA with to the consensus sequence 5'-T[GT]ACGT[GA][GT]-3' and has transcriptional activation activity from UPRE. According to PubMed:11956138 and to PubMed:16925989 binds to NF-kappa-B site and has transcriptional activation activity from NF-kappa-B-containing regulatory elements.