Except the basal, of course.
That’s a lab test, not something you can buy like a glucometer or ketone stick for ad hoc measurements at home. Sorry, but you should forget about measuring that.
It’s unlikely that the cause is insulin somehow hiding out in the body unused and then suddenly becoming active. The observation you report suggests the insulin dosing needs tuning.
The observation “insulin had zero impact” just indicates there was insufficient insulin. This could be from insufficient basal, or from too weak an insulin:carb ratio. The standard advice is to tune the basal first, to provide a stable foundation that makes it easier to understand cause and effect when analyzing BG excursions. Unfortunately, tuning the basal is kind of like tuning a guitar: you can get it perfect, but it won’t stay that way.
The observation “suddenly her numbers crashed at 4pm each day” makes me think there’s too much basal. If it were too much meal bolus at lunch, there could be a drifting down at 4pm from the tail of the insulin action (for Novolog and Humalog the actual duration of insulin action is around 5 1/2 hours—even though pumpers tend to use shorter numbers, when using algorithmic closed-loop control it can become important to use the true value.) But you describe it as a crashing at 4pm, which sounds like too much basal for the circumstances. (Is someone running around a sports field at 3pm? Exercise increases sensitivity, so basal generally needs to decrease for exercise.)
If I were quietly at home and my BG crashed at 4pm each day, I would decrease the basal a small amount starting at 2pm or 2:30 and would expect that to reduce the speed of the crash the next day. I’d continue decreasing a small amount each day or second day until the decline no longer happens. For me, adjusting the basal by “a small amount” means 0.025 units per hour. Like decreasing from 0.25u/h to 0.225u/h.
Back to the question you originally asked (amended to call it DIA.) The duration of insulin action should be handled as a constant. Don’t think of it as varying depending on time of day or activity. For general pumping, 4 hours is a reasonable number. It’s really not a useful knob to turn anyway. (Except for people trying to run the 670G pump in closed loop. They adjust DIA as a desperate work-around for the limitations of that system.)
The first parameter to adjust is the basal schedule. The goal here is to adjust it so the BG doesn’t tend to rise all by itself or fall all by itself in the absence of other influences. A good basal schedule tends to be stable for a few months, maybe changing with the seasons. A principal exception is in women who are cycling monthly, look up T1Allison’s thread on that. The basal schedule is a bit elusive because “absence of other influences” almost never happens. So the goal here is to set a basal schedule that doesn’t show a regular pattern of drifting low or drifting high at the same time each day, even if a meal is skipped.
Once the basal schedule is set, then we routinely use temp basals to handle the changes in insulin sensitivity that our bodies experience during exercise, illness, hormone spikes and so on. Some people set temp basals several times per day, to prepare for exercise or to help handle unexplained BG excursions that we blame on “hormones”.
The insulin:carb ratio is fairly stable in my experience; it drifts quite slowly for me. Dialing it in can be a challenge even after the basal schedule is good, because meal-related BG excursions have to do with two factors, namely the amount of insulin and the timing relationship between the speed of insulin action and the speed of digestion (which changes greatly depending on the composition of the food.) Speaking in broad-brush, if the mealtime BG curve shows too high a spike initially, but then the BG goes low around 2 or 3 hours later, you wanted a smaller dose of insulin but given earlier before the meal (“pre-bolusing”). The digestion released carbs into the bloodstream before enough insulin was there to handle it, causing a rise, but then there was too much insulin that hadn’t been used up by the amount of carbs eaten. A different pattern is the BG that rises but then plateaus rather than coming back down. This suggests that more insulin was needed. Could be from too weak an insulin:carb ratio, or could be from a meal heavy in protein and fat, which releases additional glucose into the bloodstream over a period of hours. (We generally handle that by a “dual-wave” bolus that gives enough up front to handle the carb spike, and continues to deliver additional insulin for a couple or few hours to handle the extended digestion.) So anyway, the goal of adjusting the insulin:carb ratio is to remove a regular pattern of “I always end up high after eating” or “I always go low after eating”.
The third knob (after basal schedule and bolus strength and timing) is the correction ratio AKA insulin sensitivity. In practice we select a number that generally seems to make corrections work ok, and then we don’t change that number. Do corrections always seem to cause a crash? Weaken the correction ratio. Do corrections always seem to leave our BG too high? Strengthen the correction ratio. But we’re going for “generally right” on this parameter, because in practice, corrections rarely work right because of so many confounding factors. My practice is that I watch the CGM and make additional adjustments with insulin or carb to steer the BG in a good direction based on clues I see in the graph. (This practice is called “sugar surfing” after the title of the book by Stephen Ponder.)
I’ll stop now. It looked like OP was seeking novice’s guidance, but I have a tendency to induce TL;DR.