|Tris sees Four's tattoo for the first time right after she has been in Four's Fearscape - and so do we|
We have seen Marcus come at Four with his leather belt to beat him in multiple selves - a nice wave to The Matrix. We now know that Marcus has beaten Tobias/Four since a young child and Four is paralyzed with fear of him as he regresses to a young child when confronted by him in his Fearscape - (my new word for it.)
We are not told any more details, but the scene is allowed to resonate in our minds.When you are beaten your body/mind is inscribed as the two are inseparable, in a relation with each other.
We know Tobias's back was beaten because the welts and wounds would not be visible there.The tattoo is announcing the beatings to the world if the world sees.The beatings are revealed and concealed.The belt sliced around to his arms where the tattoo becomes stripes.
A tattoo such as this INSCRIBES the body/mind again. The MEMORY of the constant beatings is inscribed. The memory produces CONSCIOUSNESS of the original inscription. As repetition reinforced the memory of the pain, the tattoo reinforces the memory of what it emotionally and mentally means to you.