Like I said, it's possible that 4U is too high a dose for him. It's hard to say how low the dose is taking him without any midcycle tests.
Is he still getting dry food? If he has been fully transitioned to low carb wet food, his insulin needs could be much lower.
You want to get mid-cycle tests but failing that you might want to look at a lower dose to keep him safe.
Do you use syringes or a pen to give insulin? With syringes you can give doses that are fractions of a unit for e.g 3.5U. With a pen, you can only dose whole units.