4 month sleep regression

Hi! My baby is turning 4 months in a few days but it seems like sleep regression has started. He used to be able to self soothe to sleep for his first two naps of the day when I put him down awake. But for all naps and bedtime now, he would fuss the moment he is laid down in the cot. Bedside soothing isn't working out too.. we've been carrying him until very drowsy before we put him down in the cot. Was wondering if any experienced parents have done gentle sleep training and how did you do it? how long did it take?