Given that the car consumes 6 liters of gas for every 100 kilometers it covers, then it needs to consume 4000(6)÷100 = 240 liters of gas to cover 4000 kilometers.
Since the cost of gas is $1.50 per liter, then 240 liters would cost (1.50)(240) = $360.
Since all of four friends would share the cost equally, then it will cost 360 ÷ 4 = $90 each person for the trip.
Answer: $90
