(((Melissa))) I can feel your frustration. Look at my Lucy's spreadsheet - she went up and down in dose a few times before she finally decided to settle in good numbers and eventually go OTJ. I would do dose reductions too quickly or before she was ready for them, and when I did too many dose reductions she would lose momentum and her numbers got worse again. Then I had to start increasing the dose again. In Lucy's case, her first breakthrough dose was 4.25u. When I had to start increasing again, again she had to go all the way to 4.25u to get another breakthrough. Sometimes, when glucose toxicity starts to set in, we see cats that have to be increased beyond their original breakthrough dose before things kick into gear again.
I remember that time with Lucy very well. I tried different food, I got new insulin, I even got a new meter and test strips thinking I had somehow damaged those! But it was just that her insulin needs had changed. It's frustrating, I know.
Celle has had breakthroughs at around 4.5 units before, so I don't think you are out of line. Also, if she was over dose you would be seeing some signs that she is dropping at times, but she actually looks pretty flat so I don't think you are missing any action. It looks like she started losing it when you started reducing at 3.75u. Usually the most successful course of action in this case is to just keep increasing the dose according to protocol until you hit a new breakthrough. Be prepared that when she does hit that breakthrough, she will probably start needing dose reductions again. You can test and you are capable of handling her numbers, so you'll be fine.
You can think about testing for IAA if you want to. At this point I don't think there is enough data to make me extremely suspicious, but it is a possibility and would also explain her patterns. I just know, from personal experience, that it is possible for a cat to have this particular type of pattern without IAA too.