Your laptop power supply is in 99.99% of cases a switch mode unit.
In a low power switch mode power supply, the mains inlet is followed by a filter, then there is a rectifier to convert the AC to DC at high voltage. This is followed by the switching bit which transforms the DC to high frequency AC and a transformer to convert it to low voltage AC, which is then rectified again.
The input filter and rectifier are the only bits which get exposed to a frequency other than your mains frequency at home.
The filter is there to stop the power supply from polluting the mains supply with high frequency noise generated within it. It will typically be a low pass filter with a cut-off frequency well above 400 Hz.
The rectifier is typically just a diode bridge which will not suffer ill effects from being exposed to 400 Hz. I cannot think of any modern diodes used in these type of units which could handle 60 but not 400 Hz.
Bottom line is that as long as your power supply is specified for 115 V operation (most of them are 100 - 265 V) it will work just fine, as many people on here have reported before. No transformer will get exposed to a frequency it was not designed for.
Hope this helps.
Gerard (power supply tester for a computer manufacturer in a previous career)