sleep training with the 4 month sleep regression

Our son was doing great with naps and sleeping at night and then today it was like another child was here. Hello sleep regression! We have been doin a routine but need to be more consistent with it. Any moms sleep training have any advice? Fever method successful and how long do you let them cry? Cause I don't want him to feel abandoned or alone. I want to help teach him to self soothe.