Four Cell v. 5 Cell
The following statements can be made, as per a discussion of 4 v. 5 cells for receiver packs with Red’s Battery Clinic:
4 cells cost less
4 cells are more reliable than 5 (more parts/connections = less reliability)
4 cells give you longer flying time than 5 cells of equal capacity
4 cells are lighter
4 cells put less stress on radio and servo components (unless regulators are employed)
5 cells give you faster servo response
5 cells allow for voltage regulation to give you constant servo response.
5 cells give you a margin of safety if one cell shorts and is not detected in preflight procedures. Failure to detect a shorted cell will eventually cause cell leakage with wire corrosion that will ultimately end up as an open circuit.
Usually the same charger that comes with your radio system will accommodate both 4 and 5 cells, for 5 cells the charge may take slightly longer. A standard Futaba Charger will charge 5 cells at about 40 mAh, whereas it will charge a 4 cell pack at 55 mAh.
All present day radio systems seem to accommodate 5 cell receiver packs without problems.
Weighing the above the modeler can make a decision. Is he sufficiently skilled to notice the difference in servo speed to justify the cost/weight penalty of a 5-cell pack?