Vivian, I LOVE these tux shirts. They make all my outfits current - and I reach for them ALL THE TIME. Yesterday I wore the white during the day and the blue out at night. I have three of these shirts. They are my basic t-shirt, and sometimes I want a fourth one.
Okay. I needed to ramble before I got to your point. These shirts are boxy. The model is wearing the shirt much tighter than I do. I almost never wear the shirt untucked - always tucked and sometimes partially tucked - which happens naturally at the back. And its slouchy no matter what. I scrunch the sleeves for structure. The shirts crease a lot because its stunningly soft and crisp cotton. So there are my warnings. I vote get both the 4 and 6 and assess which fit you prefer. I love them big and boxy. You might like them a little more fitted.
These are my favourite shirts at the moment. I adore their masculine vibe and refined quality. Hope that helps.
