The standard width of a double car garage is 20.5 feet. That is a big enough size to fit a large sedan and a truck inside.
Normal width of a single car garage is 8' feet. Normal width of a double car garage is 16' feet. These answers are for doors typically found in the USA.
The average with of an one car garage is 12 feet. Many people will want a few extra feet of width for storage.
20 feet
Typically, a 2 car garage minimum width is 24'. This said, there is no reference to minimum widths for garages in the IRC building code that I can find.
Generally 8' or 9'
a little wider than 2 cars
A standard 2 car garage is 24 ft x 24 ft. So you would double that for a four car garage.
You'll need to get a nonstandard garage door (not single-car width), so that may be very hard to find.
Sixty feet is a good size width for a 3 car garage. This comes from the Illustrated Guide to Tactical Drawing: How to Determine Floorplans. It calculates the size of the cars with doors open and adequate space to walk between vehicles.
8 or 9 foot, either one is common.
Most are mot big enough and most are around 24x22. A much better size for a 2 car garage is 30x28.