I think there may be too many changes. (Some cats can do well with a sliding scale; some can't as they take more than one cycle to really react). He drops well midcycle but does seem to bounce for the next preshot. As soon as you can get a curve, you can see if there is a lower number earlier than +6 or later than +8. My thinking is that he was still dropping at +7 yesterday - maybe he was lower later? The good news is that he doesn't bounce into the 400 range. I think maybe 3.4 was too low for your nice yellow, but it is always better to be safe than sorry.
It is difficult to know whether it is too much or too little insulin, unless you get a huge drop into the 50s. Then we know it's too much. Sometimes if they are high and flat and the dose was rapidly increased, we think about a restart. But you have increased carefully and have seen some lower levels.
If he were mine, I'd stick with 4 units for 3 days, getting a curve in you can and see what things look like. (Unless you get a 200 preshot. Then get advice about reducing). Get a night nadir every so often like you have been.
The good news is that he does drop into nice blues. He may be in good renal ranges a good part of the cycle (under 250ish) his highs are not outrageous. Just keep collecting data, post and we will continue to help you out. He is tricky as he has some medical issues and meds, but I think you are making slow, steady progress.