Welcome to the FDMB Alex and extra sweet Nemo!!
It sounds like you're having the same issues a lot of our new members have when they come here...they've listened to their vet and their vet is out of touch with the latest treatments for diabetes. Luckily, you're only concern right now is Nemo still being high....too many come here when their cat is having a hypoglycemic crisis. 4 units is a huge dose unless Nemo has been diagnosed with a high dose condition, and it doesn't seem like that's true at this point.
First, you will need to get more testing in. At the very least, you need to ALWAYS test before shooting, and then during the AM cycle, at least 1 test somewhere between +5 and +7 (5 to 7 hours after the morning shot) and then a "before bed" test on the PM cycle. The more tests you can get in, the sooner you'll learn how Nemo reacts to each dose.
We need to see how LOW the dose is taking Nemo (the lowest point is called the nadir). I'm not as familiar with Pro Zinc, although I do know it's one of the 3 insulins that does work well in cats, but I think I'd suggest you go back to maybe 1 unit every 12 hours and get as many tests as you can in so we can see how low he's going. That big of a dose MAY be taking Nemo into hypo numbers, and then Nemo is "bouncing". We only do increases in .25 unit increments, never whole units, so it looks like your vet has gone too high, too fast (something we see a lot of here unfortunately).
Bouncing happens when the blood glucose goes too low (or lower than the body has become accustomed to) and the liver freaks out and releases hormones and sugars to bring it back up fast. When our kitties were "in the wild", this was a survival mechanism for them. It's totally possible that although his pre-shot numbers are in the 300's, his mid-cycle numbers are much lower, his liver is kicking in bringing him back up fast, and then you're seeing another high pre-shot number.
We have a great spreadsheet program to help keep track of Nemo's numbers too. Here's
How to get a spreadsheet so you can get one started. Pretty much everyone here has their cats spreadsheet link in their signature so others can look to see how the cat is doing and help with advice on increasing or decreasing.
Keep asking questions! There are lots of us here managing our kitties diabetes at home and doing a great job, and we can help teach you how to too! Not only will you save money by not having to pay for vet tests, but you'll do a much better job of caring for Nemo both safely and effectively!