Slopes in pipes and drains are decided by quantity of water or wastewater to be transported through it by the action of gravity in cumecs. Following things influence the slopes of drains and pipes:

1. Quantity of water to be transported

2. Size or diameter of the pipe

3. Distance the water to be carried

4. Velocity or the pressure required for the water at the outlet, if any

Slopes of drains and pipes at construction site is checked by dumpy level or level machines. For the same, you have to calculate depth of pipes at different intervals.

Suppose, 1:4 slope of the pipe is required as per drawing (as mentioned by you), then the depth of pipe at should have a difference of 1 m from start of the pipe to the end of pipe at 4m. If you want to know the depth of pipe at 2m length, then it will be half of the end position, i.e. 0.5m. These are calculated based on triangle.

Slopes are always mentioned w.r.t. a reference point. Consider your reference point as a levelled surface. Then length of the pipe required will be length of the triangle, base will be the depth required from the start to end of the pipe. Now you can divide the triangle at different point and calculate the depth required at different locations of the pipeline or drain system.

I will provide your more details with figures soon.