、。
I'm supposed to meet him at four.
Literal
I [topic-は], 4 o'clock [at-に] him [to-に] meet [thing-こと-に] becoming-am.
会う takes either に or と for the person met — に frames the meeting as one-directional ('to meet someone'), while と implies a more mutual encounter ('meet with someone'). The pattern ~ことになっている = 'be supposed to, it is arranged' — circumstantial or arranged outcomes, distinct from ~ことにしている (the speaker's own habitual decision).