This depends both on the circumstances (road conditions, driver intentions in the next 15 seconds or so, etc) and the car itself. For instance, in my small petrol engined tuition car, I encourage my learners to consider being in 4th at around 25mph, if in an urban 30mph limit with few obstructions and good views. However, in my medium sized diesel, I will often be in 3rd in those circumstances.
Incidentally, using 4th at 15mph will stall many cars, and will certainly result in severe inflexibilty from the engine. I would also expect that to result in a minor driver error on test in many instances.
