Hi there. I think the problem may be not enough A times. If he's not tired enough for a long nap, no amount of W2S is going to help, unfortunately.
At 4 months, he can likely handle more A time. I'd shoot for 1.5 hours in the morning, so first nap starting around 8:30 and second nap starting 1.5-1.75 hours after end of first nap.
Do you have a consistent wind-down routine? Use white noise? Darken his room?
Keep up the good work - you're doing a great job so far and congrats on ending the accidental parenting!