Marje is thinking 4.0u - partially because of his neuropathy. Wouldn't want to make it worse by sending him up into high numbers.
I'm thinking 3.0u, mainly because you can only test til +4 tonight. But, if he heads back into low numbers after his shot then you'll have to monitor until he comes up. That could happen no matter what you shoot - we don't have any way to know. With Lantus, often shooting a normal number like this you can have a flat cycle, but your situation with the dry food leaving is a little bit different than a cat that's already been on canned low carb, has arrived at the "right" dose appropriately and the cat is then getting the regular dose.
You can also drop the dose lower tonight, see what happens in the next cycle and come back tomorrow with a "new normal" for the dose. He's not dropping, though, so I'm not sure if I would do that or not.
There isn't a clear path to just say "do this and then this" when a cat has had a higher dose but the circumstances have changed and you need to somewhat restart the dose.
You've got quite a bit of experience with him, even though the Lantus is relatively new to you. You know his history and how he "works." The choice is up to you, but those are some things to think about. What do you think you'd like to do?