Your friend is wrong...White Rocks are not bred for maximum size~the breed has been large since the beginning and has not been "bred up" to a certain size. As such, they do not normally have leg problems or growth issues. White Rock is a variety of the Plymouth Rock breed and have the legs to support their large frame, so are not prone to leg issues at all. They are my favorite breed and I'm very familiar with their growth and characteristics.
Now, Cornish Cross chickens might be accused of having leg problems due to quick growth but this one is hardly large enough to have leg problems just yet. Yours appears to be a CX, instead of a White Rock...sometimes people refer to them as White Rock Cross but they are not anything close to being a White Rock bird. If it is a CX, it is small for 4 wks, so I'm thinking quick growth is not the issue.
This is a pic of a 4 wk CX that has been free ranged and not fed unlimited feeds and he still appears bigger than yours, so I doubt leg issues are the problem.
The legs don't appear to be broken...that would be easy to ascertain by a simple palpation of the bones and joints.
Here's a pic of a bird with Mareks but it is usually asymmetrical in the leg paralysis...this one looks eerily like the positioning of your bird. Does your bird seem to have trouble breathing? Any diarrhea?
I'm sorry I couldn't be more help as I've never had illness in my flocks these many years, so have not had experience with diagnosing diseases, but there are thousands of folks on BYC (BackyardChickens.com) who have experience with poultry illnesses and can probably help you more with this.