Since this is your very first day giving Jack insulin, and you already got a BG of 67 at +4, this 0.5U dose may be too high. Lantus is a depot insulin, so the dose you give in one cycle does not all go to work immediately. Once the depot builds up, you will start to see the full effects of the dose you are giving. This usually takes 3-5 days.
With that steep drop from 309 at AMPS to 67 at +4, I'm concerned this 0.5U dose may be too much for Jack, especially with the cumulative nature of the dose.
Are you able to monitor this evening? Testing at +2 would be a great idea. If Jack is a lot lower at that time, you may need to monitor through the night to keep Jack safe.
If it drops too low, you need to give some higher carb food to raise the numbers up. If that happens, go back to your very first post, click on edit,change the post icon to the 911 icon, and submit the change. Then, add a new reply to your post and let people know what is going on.
It's only the very first post where the heading and posting icon change and show up on the board. Otherwise, it's buried way down in the psot, and those of us that scan the board may easily miss that you have a urgent need. If there is no one on here on the Feline Health forum, please post a new topic with a 911 icon over in the Lantus TR group. Lots of experienced people and west coast people there, up much later than us east coast folks. There would be someone there to help you.