First of all it has everything to do with arduino. My project is completely based on an Arduino board.
+1 on running 3s directly.Want to avoid the 9v complete discharge voltage? Add more capacity by running multile packs in parallel like you mentioned. For the same run time, you will only pull half the capacity of each pack.How long are you looking at running this?
The question was different. What is more efficient for the project either to have 3S and one step-up module or 4S and a step-down buck?