I don't claim to be an expert at chicken sexing, but that chick's comb is barely visible, I have a hard time believing that's too large a comb for a 4-5 wk old pullet. My RIR roo chick is almost 6 wks old and his comb is quite red today, and it started turning dark pink 2 weeks ago. Here's a picture of him that I took earlier this afternoon.
The chick in your picture looks a lot more like a Red Sex-Linked pullet than a RIR roo in my opinion.

The chick in your picture looks a lot more like a Red Sex-Linked pullet than a RIR roo in my opinion.