In what sequence were the Monasteries built?

As per Xuanzang’s description there are three monasteries in the row North-East of Monastery of Tathagata Gupta i.e. monastery no 1A but the excavations suggests based on the dating technology that there must have been four monasteries in that particular row at the time of Xuanzang visit. So, it is still a mystery to be solved about the sequence in which these were built.