Not that I will advocate cheating to get a too-short child onto a ride, but if we are talking about 1/4 inch, I think that there is a lot of latitude for it being accounted for just by posture. CM's are strangers, and many kids that age will cower when spoken to by a stranger.
When DS was just at/over 40", we had a terrible time with his height measuring differently every time he was measured. In some cases, he rode and wanted to ride again, and the second time was told he was too small, by the same CM, no less. We learned a couple of things about measurement on that trip.
If there is a ride that is just borderline, be sure to do it first thing in the morning. People of all ages actually get a smidge shorter as the day wears on, partly from gravity over the course of the day, and partly from a certain droop factor caused by fatigue. In a borderline situation, the child will usually measure a little taller in the morning.
Watch the positioning of your child's feet when he is measured, especially against a permanent signpost. DS tended to put his tush against the sign and then balance by putting his feet either too far in front of him, or too far to the side. Make sure the child's feet are centered directly beneath his shoulders, and that he is not putting weight against the post, b/c if he is leaning on the post, he'll be just a little bit shorter.
Lastly, and most important, do not pick up or carry your child in the rideline or as you approach the ride. The CM's may or may not measure a walking child who appears to be tall enough, but they are always going to carefully measure any child that they see being carried. We found that when DS was right at 40" that he was almost never measured if he walked. He is very thin and lanky child, and he gives a visual impression of tallness; we found it worked best to let that work for him, rather than having to cajole him to stand up straight when he was cowering away from the "stranger" who was trying to measure his height.