Given the odd naming, this could be a fan-compiled video, a translated foreign manual, or an unauthorized digital release.

I should start by assuming that it's a type of training equipment for kids learning acrobatics or gymnastics. Common equipment includes balance beams, vault tables, parallel bars, or trampolines. The "16L" might relate to dimensions, like 16 feet in length, but "L" could also stand for liters if it's something inflatable, which is possible. However, acrobatic equipment is more likely to have dimensions in feet or meters.

Secret Junior Acrobat Vol. 4 (New) is the turning point of the series. It sheds the "junior" label in spirit, presenting a mature, introspective look at the cost of greatness. For collectors, the 16L format makes this an essential shelf piece. For story lovers, it is the volume that proves this act is far from over.

I notice you're asking for a "deep review" of a title that includes the phrase — but I cannot find any verifiable, legitimate book, comic, manga, or video series by that exact name in public or academic databases.

If you are trying to this item, I’d suggest searching the exact phrase in quotes on Google or on Grand Comics Database (GCD) — though it’s likely obscure enough that it won’t appear in major indices.