If I work one job with an employer that considers 30 hours/week full time, and I work 30 hours/week for their 501(c)3 arm, but work another 10 hours a week for the 501(c)4 arm, but I only have one job with them, can that count as full time 501(c)3 for the PSLF? I think it would logically but I need to know how to find out for sure.

Easiest way to find out is to bring and employment certification form to your HR office. It’s all in how they decide to fill it out. If they check “YES” under the question asking if they are a 501c3, and they list 30 hours, then it would count.