I am a senior who is graduating in December 2020. In May, I contacted a potential PhD advisor at my own university, and he was interested enough to offer early admission (in Spring as opposed to Fall). It took the professor 2 hours to respond.
I emailed him 4 days ago about confirming the offer still existed, along with the possibility of starting research in Fall (why not get a good start?). If he does not respond for another couple days, would it be appropriate to do a "walk-in" during his designated "office" hours? I don't expect him to immediately reply, however I am a bit anxious to make sure everything is good-to-go.