4 inch. We bought an 8 inch one when shutdown really started to drag on, because it was bought to go on tiles and the website recommended the extra height if it wasn’t going on a soft surface. The height of it is offputting for my gymnast as it makes her more worried about bouncing off the side (which she won’t do as it is two meters wide, but she is a very cautious kid), and in hindsight I would have bought the more slimline one.
We also have crash mats at one end, a strict one way tumbling rule, and she is only to use it for skill maintenance, not working new or not yet mastered skills.
Honestly, it is actually most useful for non tumbling - doing the ‘lines’ they always do in gym - straight jumps, long jumps, handstand rolls, split leaps and jumps etc.
The bounce is different, although she came from a gym where airmats And tumble traks are used in off season to protect joints, so it was at least familiar to her.