Hi Nico! The hot water tank was designed with dimensions and diameters studied and tested, modifying something always brings some fault or risk. You would be modifying the output rate of the burned gases inside the temperature exchanger. What you can do if the output of the pipe of 4 goes up is to put pipes of 3 and use the old jacketed. If there is any elbow, check if you can remove the other sections and assemble your 3-pipe from that elbow to the device and then to the outside, getting the 3-channel into the old one in its entirety and thus respect a diameter of 3 from the beginning to the end, without narrowing or sebredimensionados.