Those who followed the Imam in his 5th while they know it is 5th, their prayer is void!

People should ignore the Imam and complete their prayer on their own if he stood to a fifth and wouldn’t listen to them when they said subhana Allah.

If a person prays in congregation and the imam prayed 5 rakat instead of 4, do we people behind also pray 5 or 4? Because if we do 4, when do we do taslim, after once imam is done?