First of all, please make sure your SD card is compatible with the H4n with the list below.
Next, plug your H4n into your computer via USB with the unit turned OFF. After doing so, the H4n will power up and display the USB menu where your options are “Storage” and “Audio I/F.” Select “Storage” by pressing the jog wheel. You are now in storage mode and your H4n will act as an SD card reader.
***Before unplugging your H4n in Storage mode, please be sure to safely eject the device so that you do not damage any files stored on the SD card.