<image>Many cities in our country face water shortages. To enhance residents' awareness of water conservation, a certain city has established two pricing standards for monthly water usage: within 4 tons (including 4 tons) and above 4 tons, where the price per ton of water is different for each range. The monthly water fee y (in yuan) that a user should pay is a function of the water usage x (in tons), and the function's graph is shown in the figure.

(1) Observing the graph, derive the function's expressions in different ranges;
(2) State the pricing standards of the water company within these two usage ranges;
(3) If a user pays a water fee of 12.8 yuan for that month, determine how many tons of water they used.