Putative guanine nucleotide-exchange factor (GEF) involved in the formation or budding of transport vesicles from the ER. Positive regulator of SAR1 probably through inhibition of the GTPase activation by SEC23 (By similarity). In the process of transport, SED4 itself may migrate to the Golgi apparatus and function in subsequent transport events. Belongs to the WD repeat SEC12 family.