Strictly speaking, I would argue that only the first one is correct. You are referencing a definite "next time" you will be in the city, so you want a "the", and you are specially referencing a time when you are in the city, not a future time, so you want present tense. In other words, at that "next time", being in the city is a present-tense activity, not a future-tense activity.
However, to my ear (native American English speaker, born and raised on the west coast), all four sound pretty natural. If I were to write one, I would probably use the first construction, though I'm sure I've used similar constructions to all four at one point or another. You definitely wouldn't be misunderstood with any of the four constructions.
As a note, when spoken, I would definitely expect "I am" and "I will" to be contracted. They sound very stilted when fully spoken.