I would assume that security at T1/T2/T3 is going to check your boarding pass, and if you are meant to be departing T4, you won't be permitted to enter.
Since T1-T3 are all connected airside, one can enter the immigration point using a different terminal from the departing flight. That was my experience but YMMV. Maybe they want to segregate T4 for the time being. AFAIK, even normal airport pass holders have to be T4-approved to be granted access to the sterile areas unlike the current operations of T1,2,3.