Final answer:
The maximum fixture units for a 4-inch horizontal drain with a 1/8 inch per foot slope is not provided but can be determined by consulting current plumbing codes or standards, which consider pipe size, slope, and flow rate, including the effects of devices that increase flow rate.
Step-by-step explanation:
The question pertains to the plumbing principles for determining the capacity of a drain system, which is generally covered in engineering, especially in topics related to civil or sanitary engineering. The number of fixture units that a drain can service is based on its diameter, slope, and the expected flow rate. Although the specific value is not given in the response, it can typically be found in plumbing codes or standards which provide tables and formulas for calculating the permissible fixture units for different pipe sizes and slopes.
It's important to consult the current plumbing code or a professional reference to determine the exact maximum fixture units for a 4-inch horizontal drain with a 1/8 inch per foot slope. These codes take into account the increased flow rate that can be achieved with certain devices installed in the drains, as hinted by the mention of Figure 12.28, which would presumably show a device that augments the flow rate in the drainage system.