The difficulty at this point is that you haven't had a chance to accumulate a lot of information. I do think it makes sense to reduce the dose as you noted earlier to 1.5u. It's much easier on your nerves to not have to deal with unexpectedly low numbers when you're starting out on a new insulin. I think there's a good chance that 4.0u was way too much Vetsulin, as well.
What I would suggest is that you get at the minimum, 4 tests. You want to test at pre-shot like you've been doing. You want to get at least one test each cycle, preferably in the neighborhood of Nick's nadir. Vetsulin tends to drop numbers early and fast. Lantus is very different. Lantus doesn't begin to start working until about 2 hours after an injection. For most cats, nadir is somewhere in the vicinity of +6. You may not see much if you're testing at +1, especially if your pre-shot is high. (It's a different story if you have a low pre-shot and you want to make sure your cat stays in safe numbers.) In other words, when you've been testing at +1 during the PM cycle, it's not giving you much information about where numbers are heading at night.