When / how should I ask about a manuscript's status in review? I have a manuscript that has been in review for 60 days. The journal asks for reviews within four weeks. At what point is it acceptable (and not detrimental) to contact the editor for an update? How should I phrase the request?
I don’t know your field, but in mine (theoretical computer science), you have to get used to long review processes. My personal rule is to contact the journal six months after submission. Given recent events, my new, additional rule is to also state upon submission that I can suggest referees if need be.
As EpiGrad commented, the four weeks time certainly refers to the time allowed to the referee. But additional stages take longer: a first quick scan of your paper to decide whether it’s worth troubling an editor with it; then the editor has to find referees; and, assuming they all complete their reviews in time, the editor also needs a little bit of time to reach a decision based on the reviews. Moreover, your paper might not be their top priority.
As for the “how”, something like this would do:

Dear Editor,
I understand that you must be extremely busy, but I was wondering about the status of my manuscript [title, reference number] that I submitted on [date]. Have you heard back from the referees yet?
Thanks a lot for your time.
Best regards,
[name]