Can you measure the hardness of your water (TDS meter, or GH/KH kit)
Adding 20% water at 7.2 to a 4.6 is not that bad, the fish should be ok. But I am concerned about your 4.6 to start with
if your water is soft it will have a low pH.
so if it's coming out of the tap at 8-9 (assuming it is indeed soft, which it may not be until you measure it), probably that your city is adding NaOH, you could ask your city to confirm.
I'm not a fan of canister filters, as they are just a giant trap for gunk. Maybe try doing a really good cleanup of it, or do a test running your tank on a air-sponge filter only (the goal would be to see if your canister is causing the low pH or not). You could add crushed coral to your filter or tank to help buffer the water quite a bit and that would keep the pH higher.
All that said 4.6 is very low. Discus can deal with wide pH ranges, but I would like to figure out why it goes so low.
To put it in perspective, here in Seattle area, the water comes out extremely pure (TDS <30 ppm). They do add NaoH to the water, so it comes out around 8.5. Once in the tank, it settles at 6.8 after a few hours and stays there forever (bare bottom, running a sump with porret foam, and filter rocks, clay, etc). I have ran these tank on air-sponge only with identical result.
I don't age my water, and I do a 90% weekly, here is a graph of my pH fluctuation. the peaks are when I do water changes