HIIT versus treadmill runs

It's going to be calorie balance that manages your weight, not exercise. If the HIIT burns the same number of calories as the treadmill, and you keep eating at the same calorie level, you're all set.

A potential sticky thing is that calories are hard to estimate for most of the many, many things that are called HIIT these days. Many of them are fast-paced bodyweight circuits, calisthenics circuits, or other circuit-type exercise, stuff like that. Those raise heart rate for reasons that are partly related to oxygen uptake (the thing that really correlates with calorie burn to some extent) and partly related to certain kinds of stress (strength pieces, arms overhead, unusual breathing patterns because of the nature of the exertion, etc.).

That means that a fitness tracker or heart rate monitor is likely to give distorted calorie estimate. The calorie estimates provided by the exercise purveyor may be on the high side as a marketing move, plus they don't usually take into account individual effort/intensity levels, body weights, etc.

If you stick with this new routine for a month, eating exactly as you have been, you'll have your answer, or close enough, in the scale results. As long as you keep eating managed, it's unlikely that you'll gain any meaningful amount of weight from the difference, as a worst case scenario. Any 2-hours-per-week activity vs. any other activity of the same duration that feels loosely similar, is probably not differing by zillions of calories, but maybe something in the low hundreds per week at most.

Even if it turns out that the treadmill is higher burn than the HIIT, it's unlikely to be a big enough difference to be a problem, IMO. Let's say there's a 200 calorie difference per workout (unlikely to be anything like that big, IMO, in reality). 200 x 4 times a week would be 800 calories. If you burn 800 calories fewer per week, it'll take over a month to make a pound's difference in body weight. It's unlikely to be that big.

If you're maintaining weight, that small a gain might be visible on the scale in a month. If you're losing weight at some reasonable rate, it's likely to be lost in the noise of day-to-day weight fluctuations.

A thing to watch out for, though, IMO, is if you find the HIIT much more tiring: Moving less during the day from fatigue (during non-exercise time) is potentially a higher-magnitude impact than the calorie difference from the exercise.

Someone may come in and tell you that the EPOC (excess post-exercise oxygen consumption, or "afterburn") from the HIIT will be a world-beater, make the HIIT burn lots more calories than the treadmill. If one does the math on that sort of thing, the reality, in calories, tends to be underwhelming.