A couple of thoughts...
First, I wouldn't take one day's worth of tests and make a decision about dose. I don't think you mentioned how long you've been giving Muffin insulin. With Lantus, you need to establish a shed -- what amounts to a storage area under the skin. It takes 5 - 7 days when just starting Lantus for this to build so that all of the insulin you are giving is available for use. So, it takes roughly a week for Lantus to really start working. According to the dosing protocol we use, you hold the initial dose for 5 - 7 days. Changes in dose are then made in no more that 0.5u increments.
Lantus is a 12 hour insulin. I'm not sure how getting a +4 test will inform your vet. A curve would be more useful since dose adjustments are made based on the lowest point in the cycle. This is most often somewhere between +5 and +7.
As others have mentioned, dry food will drive numbers higher. Depending on the carbohydrate count, dry can elevate numbers to a very high level. The vast majority of cats here eat low carb, canned food.
I would encourage you to read the starred, sticky notes at the top of the Board. In addition, the notes will link you to other information on food, formal versions of the dosing protocol, our slang dictionary, symptoms of hypo and what you need for a hypo kit, etc.