Welcome and, no, charging the batteries inside the mod is not safe regardless of which USB charging method you use because you run the risk of overcharging the batteries─as there's no balanced charging in the mod─and the heat buildup also poses some additional risks. Just grab a separate 4-bay battery charger (such as the Nitecore NEW i4 or something very similar to it), and, keep the charger on a non-flammable surface like, for example, a ceramic tile at least until the charger has finished charging all the batteries.