You will need to run AWG #4.
A #1 copper wire with an insulation rating of 90 degrees C is rated at 140 amps.
Depends on the size of the sub-panel in that garage. If you are installing a 60 amp sub-panel 400 feet away from the main service panel then use AWG # 4.
This is a voltage drop question. The voltage has to be stated before an answer can be given.
AWG #6 will work fine at 100 feet with only a 2% drop in voltage which is acceptable.
Need to know what the load amperage is on the 100 amp panel. If interested, open discuss question page.
Assuming the voltage is 240 volts, the wire size for a 100 amp panel at the end of 400 feet and holding the voltage drop to 3 %, the wire will be 3/0 in size.
AWG #8 copper will work but I personally would use #6 for that long of a run.
For that distance I would run AWG # 3 copper and install a sub-panel.
Normally you would use AWG #6 but for this long run and voltage drop I would recommend you use AWG # 4.
To answer this question a voltage has to be stated.