Your lo will be the best guide as to know when to drop to 4 feeds. If his naps start to shorten or he has trouble settling for naps and seems to want to stay awake longer, he may be ready to transition to a 4 hr routine. Also, if he doesn't seem to eat as much at 3 hours, he may be ready to wait a bit longer. It doesn't have to be exactly 4 hrs, you could do a combination of different lengths between feeding throughout the day. What does your day look like now? Do you give a df? If you try and drop a feed, and it doesn't work for him (meaning he seems hungry), you can always add that feed back.