6400 workers were employed to construct a factory in 4 years. A the end of the first year, 25% workers were retrenched. At the end of the second year, 25% of those working at that time were again retrenched. However, to complete the project in time, the number of workers was increased by 25% of the total number of workers left at the end of third year. How many workers were working during the fourth year ?