You are comparing apples to oranges. The G540 cannot draw more than 3.5A per motor. The POWER SUPPLY however must supply enough amps for total motor consumption.
A single 3.5A motor connected to a 12A PSU will only draw 3.5A of the PSU capacity. FOUR 3.5A motors connected to the same PSU will need to draw a minimum of 8.4A. At no time will the above G540 be asked to draw more than 3.5A per motor. This is far below the 7A rating of the internal fuse, so that does not apply to the PSU size calculation.