If your team needs 4 people to stay on top of all the work it does, and 1 person leaves, then you should expect to fall behind by about 25% of a day every day until a replacement is up-and-running.
1.25 days per week
5 days per 4-week month
I.e after 1 month, every member of the team will be a work-week behind on their regular duties.
This is solvable with overtime, but the opposite of cost-efficient.