My current company uses this.....
2 stripes - Junior F/O, less than 1000hrs TT
3 stripes - Senior F/O, more than 1000hrs TT
4 stripes - Captain - obviously
My previous company used only 2 or 4 stripes until relatively recently. It has now introduced 3 stripes for those F/O's recommended as Senior due to time served and good simulator assessments.
So there is no hard and fast rule other than 4 is for Captains only.