Our 4 yo just made it to 40" for our trip to WDW last week. I had read other threads that said that the height measuring sticks are sometimes a little off (might be 41") from 40" . Therefore, I told DS before the trip that we would have him measured but that he might not be big enough yet for some of the rides. Of course, at every meal for the last few months, I had been saying "eat those veggies so you will be big enough...". At every ride with a height requriement, there is either a CM with a height measuring stick at the entrance to the ride and/or a permanent sign (you must be this tall to ride...head has to touch bottom of the sign). Have your child measured before you get your fast passes. I understand that at some rides, the CM may give your child a future rider certificate if he isn't big enough yet. Last year, at BTMRR, while DH took older (and taller) DS on the ride, I took younger DS to the CM with the measuring stick and asked him to "measure" DS so that we would know how much he needed to grow to be big enough to ride on our next WDW trip. The CM was great and really made a "big" deal about how much DS needed to grow. At BTMRR this year, I took DS to the permanent sign to see if he was tall enough. The CM at the entrance watched us go to the sign, which DS was clearly taller than. When we entered the line, she measured DS with her stick. DS was standing downhill from the CM so he didn't reach the top of her stick. Nonetheless, because she had seen him stand against the stationary sign, she allowed him to go on. At Test Track this year, DS met the height requirement at the start of the line, but while we were waiting to board the next car, a CM pulled him out of the line to be measured again. DS was still tall enough. No one was measuring at Star Tours at MGM. At Kali River Rapids at AK, DS was measured and then given a yellow wristband so that we would be placed in a raft that had a special safety bar for shorter riders. Hope this helps.