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<title>Abstract</title> <bold>Background</bold> Currently, in vitro fertilization and embryo transfer (IVF-ET) typically involve transferring embryos on the third (D3) or fifth day (D5). However, a fresh cycle fourth day (D4) morula transfer offers a flexible and effective alternative. The compaction on the fourth day serves as a simple and reliable indicator to predict embryo implantation potential. The use of D4 transfer is gradually gaining popularity. Nevertheless, there is still a need for a comprehensive evaluation system for fourth day embryos.Thus, this study proposes to explore a day 4 embryo evaluation system based on the potential for the embryo to develop into a transferableblastocyst. <bold>Methods</bold> We observed the outcome of blastocyst culture for 1235 embryos from 199 patients. We considered the developmental stage, fragmentation, whether it was in a stage-specific cleavage pattern, and normal fertilization on the fourth day. The fourth day embryo evaluation system was proposed by comparing them with the effect on blastocyst formation rate. The rates of blastocyst formation with different embryo grades were compared. The receiver operative characteristics(ROC) curve was used to compare the predictive abilities of four criteria for day 4 evaluation to develop into transferableblastocysts and high-quality blastocysts. <bold>Results</bold> The developmental stage, fragmentation, normal fertilization and stage-specific cleavage pattern had the greatest impact on the formation of transferable blastocysts. There were significant differences in the formation rates of transferableblastocysts and high-quality blastocysts among different embryo grades. The areas under ROC curves of the day 4 embryo evaluation criteria constructed in this study was the highest in predicting formation of transferable or high-quality blastocyst, relative to the other three evaluation criteria. This study observed the presence of pseudo-compacted embryos similar to cleavage-stage embryos on the rate of blastocyst formation. <bold>Conclusion</bold> The Day 4 embryo evaluation criteria constructed in this study can effectively predict the ability to develop into a blastocyst.