or it could just be really run down packs. a 4s at a little over 3 volts per cell will let you for a short time charge above 6 amps on the pack. it will peter off after the voltage rises, and as the pack reaches peak voltage.

though usually it is recommended that at least 100 watts is used on 4 s pack to give optimum current as most chargers over 100 watts can use 10 amp charge rates anyways.