It is not a good idea as you dont have a protection circuit looking at each cell voltage.
You really need a circuit which checks each cell voltage and then balances the cells in the string by shunting charge current away from the cell(s) with the highest voltage, alowing the others to "catch up" (this is how it works in most lithium ion batteries used in notebooks etc).
If you don have such a circuit, impedance imbalances mean you coudl have one or more cells charging at >4.20V which is a safety hazard.
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