We have always had mattresses next to their beds so when they wake in the night we go to them and lie next to them. Trying to wean my 2.5yr old off it now! But they never come to us and always wait. You could try something similar?
I am in the same position as you! 4 month old co-sleeps and 4year old won’t sleep in his own bed. I have resulted in him sleeping with his dad. I have no advice but your not alone I would say give it a few more months and the eldest will settle x