The present disclosure provides compositions and methods for modifying endoplasmic reticulum trafficking of proteins. Also disclosed herein are methods for treating cancer and/or enhancing cancer or viral immunotherapy in a subject by increasing the extracellular secretion levels of GRP94 in the subject. Such methods comprise administering to the subject an effective amount of 4-PBA and/or a 4-PBA analog selected from among methoxy-PBA, 3-PPA, 5-PVA, hydroxy-PPA, hydroxy-PBA, and tolyl-BA.