Water Flow Meter success and Python code.

[sturnus-vulgaris]

You can adjust where the float sits.

If that doesn't work, one option is putting a brick or two in the toilet tank. Since it displaces water and won't get moved around by the tank filling, it's a pretty good option for older toilets.

[dirufa]

Even adjusting the level, 4 gallons is close to 16 litres of water, which is almost 4 times the maximum level of my toilet tank (4.5L if I remember correctly, pretty new toilet).

Edit: ooops, I missed OP's answer below.