...here's the deal on bike frames where the seat tube extends above the top tube and clamps above like your bike. The insertion needs to extend a safe distance beyond the top tube junction, not just a certain designated depth.

Otherwise, it will cam out, especially with a substantially sized rider.

So if 100 mm is 'bout four inches, looking at the photo I would not feel like that was adequate depth on that particular bike if I were riding it. If you can get 100 mm of seat post below the top tube, you're good to go.

You might get away with four inches total. I personally would not do it, but I have been known to wear a belt and suspenders on a couple of occasions.