The first word of a quote is capped if it starts a grammatically complete sentence or a fragment that stands for a complete thought. It is not capped if it is a fragment.
...Q When did you leave?
...A We left around 4:00.
...Q What do you mean "We left around 4:00"?
...Q When did you leave?
...A We left around 4:00.
...Q What do you mean "around 4:00"?
Happy punctuating!
Margie