Assuming that the water consumption of A is X tons, there are

1.8 * 4+(x-4)* 3+ 1.8 * 4+(3/5 * x-4)* 3 = 26.4

The solution is x=7.5.

Then the water fee payable by Party A = 4 *1.8+(7.5-4) * 3 =17.7 (Yuan).

Water fee payable by Party A = 4 *1.8+(7.5 * 3/5-4) * 3 = 8.7 yuan.