。
I'm supposed to meet him at 4 PM today.
Literal
I [topic-は] today [possessive-の] afternoon 4-o'clock [at-に] him [with-と] meet thing [goal-に] has-become.
~ことになっている ('it has been decided/arranged that') expresses a predetermined arrangement. This pattern indicates something decided by external circumstances or mutual agreement, contrasting with ~ことにしている which marks a personal decision. 午後４時 ('4 PM').