Personally I think HVs just say things like this because they just have random ideas, they just tell you to do it without taking a moment to ask you about your child, what they are like, what they eat, and then telling you signs to watch for to drop the bottle, etc etc etc. I'm fortunate in that my own assigned HV really does take the time to TALK with me about things, while the one I see at clinic for weigh-ins does NOT, she just throws random things out there.
If LO is still on 4 full bottles a day, and it's not interfering with solids, then I would keep the milk. Just because he is 8 months doesn't mean that it is a magical number that he has to be onto 3 bottles. Milk is far more important to him. He's on 4 bottles today, well, by the end of next week you could notice that he's dropped and is taking only half of his bottles and then you need to drop one of them - it can happen quite quickly, so just because he is a "magical age" doesn't mean anything! Even if it does start to interfere with his solids, keep in mind that milk is far more important nutrition wise at this age.
You will most likely not still be on 40 ounces of milk at age one, over the next months your LO will start to drop milk and take in more food, even if it is out of the boredom of having to sit and guzzle a full bottle - that takes 10 minutes, which for an active 10 month old, is a LOT of time that he could be exploring forbidden areas of the house, and he will take just the minimum of what he needs and jump off of your lap to get his hands in the DVD........