If she drives 400 miles today, the car will have consumed 4*400 = <<400*4=1600>>1600 gallons of gas.
Tomorrow, if she drives 200 more miles, the total distance will be 200+400 = <<200+400=600>>600 miles.
The fuel consumption by the car tomorrow is 4*600 = <<4*600=2400>>2400 gallons.
The car's total fuel consumption in the two days is 2400+1600 = <<2400+1600=4000>>4000 gallons.
#### 4000