hi there!
we give holly a small snack, like a cereal bar, or some fruit (like applesauce, strips of cutup pear, banana, that sort of thing), sometimes some crackers and cheese, or veggies like cucumber, peppers or celery. usually not much at all. if we're running late or "forget" snack then i give her only milk, usually 8 oz, which is the most milk on one go that she takes.
could you consistently at that time offer her a small somethign she likes, and some milk or water if she doesn't want milk? then put it away if it's not eaten in the next 30 min. then be flexible about tea and have it a bit earlier if she didn't snack, or a bit later (closer to 6) if she did? i'd offer the snack but not make a big deal out of her not wanting it, but then stick to your guns about snacking at 4:30 or later. maybe give her a glass of water when she does ask. you could also be happy with her not having too much food at tea and offer another small snack later just before bed, though that's never worked for us...
good luck!