I've thought about this and I finally settled to use 4 Li-ion batteries! 2 in series and 2 in parallel for increased current because it is important for me. The pack's total voltage is 8.4v maximum, so will this individual protection technique works fine or should I make a 8.4v over-voltage protection for the whole pack?
If protecting the whole pack is safe I will get it... but what IC? I couldn't find a suitable IC for this. I am glad to take help.