I have 1A* 8As and a C. I originally took 5AS levels and found it really tough. They were all facilitating subjects too(Bio, chem, Hist, Geog, and lit), so the workload was huge. I recommend taking 4 subjects at AS level due to the flexibility. Plus if you are predicted As/A*s your work ethic must be decent, that is all that is needed for 4AS levels.
4 seems to be the perfect balance in my opinion.