Unfortunately, ADNI4 was the first phase in which the RUCC and RUCA were collected

In absence of RUCA codes, Is there a way to know just rural or urban for earlier phases?

No such designation is available to the best of my knowledge.

The PTHOME field in the participant demographics table may be of some use; it indicates the type of domicile that a participant considers their primary residence; e.g. apartment/condo vs. free-standing house/home