The rules don't mention assumed sales rates within the business case presentation, but do however mention them in 1.2: vehicle design objectives. Does this mean that we use this manufacturing rate within our business presentation or go with a more realistic manufacturing ramp up period that would make more sense from a demand/market share perspective? Any ideas?

The best way to approach it is to present the most sound business case you can. If the rules assume that you will build 4 cars per day then use that, but also use any other data you have or can find that shows how many vehicles you could sell. Ex. If there are 4 million people in your target demographic and you reach just 0.1% of your demographic that would be 4,000 cars. Not real numbers but it shows the idea. You should use real manufacturing and marketing principles in your presentation just as if you were presenting a real product to a real company. Include manufacturing cost, dealer cost, retail cost, lead time, ramp up periods, time to profit, everything that someone would need to know to make a decision on whether to give you the contract.