Tetrapolis is a term used to describe a group of four cities or towns that are situated close to each other. However, there are a few synonyms that can be used in place of tetrapolis. Quadruple city is one such term that describes a group of four cities or towns. Another possible synonym could be a cluster of four cities, which also refers to four settlements situated closely. Additionally, tetrad city is another term that can be used interchangeably with tetrapolis. It's worth noting that these terms are not commonly used in everyday language but may come in handy when referring to specific historical or geographical locations.