We have that Fisher Price Aquarium too, my sister gave it to me, and the music from that always works with Jesse during the day, calms him right down and he seems to know its sleep time. We recently bought a mobile which puts lights on the roof and everything but he wont take to the music from that, only the Aquarium music settles him. We also use a dummy during the day.. or he also falls asleep in his bouncer as it has the calming vibrations thing in it (which hes done right now).. but i dont know id a 4 month old is too big for that?