I personally for sure keep 4 liquid feedings while I am nursing. While I think I can maintain a supply with 3 feedings later in the year, I just feel better about 4. Brayden was around 18 months when we dropped the 4th liquid feeding. I think we did it around the same time we dropped his morning nap since his bedtime moved up by an hour. I think Brayden needed that 4th feeding for a longer time because he just has a fast metabolism and needed that extra feeding in the day. He also would not drink more than 6 ounces per feeding, and I wanted 24 ounces of milk in him per day.
As Brayden got older, he would drink more than 6 ounces sometimes. I would give him however many ounces he needed to get to 24.
Kaitlyn is 15 months old, and we are apparently in the middle of dropping the 4th feeding. It is a weaning process for us. I am not doing it on purpose, but some days she has it, and others she doesn't. I don't count ounces with her like I did Brayden. I currently give her about 10 ounces at each meal (but she rarely drinks that much at a meal). When she was 12 months old, she drank different amounts at each feeding. Some it was 3 ounces. Others 8. As she has gotten older, she has been able to take in more ounces per meal and therefore needs that final liquid feeding less.
So with Kaitlyn, I give her 10 ounces at each meal (three times a day). Then sometimes I give her the 4th, also with 10 ounces, if she seems to need it. I can't really describe it other than I just know when she does. Part of it is that I am a more seasoned mom now and better at reading my children. Part of it is that Kaitlyn is an excellent communicator.
Kaitlyn can handle the way I do this. She monitors herself. She has also always been a small girl, usually in the 20% percentile. I am not concerned about her eating too much. She also is a random drinker. Some meals she drinks 10 ounces--others she drinks one ounce. It isn't the same feeding, either. I know there are some kids who will eat everything you put in front of them. Some kids will drink more than they need. Do what works for your child.
You can drop that 4th feeding if your child gets the liquid he needs in 3 feedings. You would also want to make sure if you give more ounces per meal that your child still eats the solid food well. You can drop it if your child continues to grow well and sleep well when you do so. I think with the 4th feeding, it is something you will just kind of know when it is the right time. Remember, if you do drop the feeding and it turns out your child isn't ready for it, you can always add it back in (thought it might be harder to do that if you are still nursing). Trust your Mommy intuition!
You can see more on dropping feedings in general on this post: Dropping Feedings