Why won’t the kit work if I end up with 4 extra spokes?
Typically, as the number of spokes changes, so does the length of the spokes, even when the diameter of the hub and rim remain the same It has to do with the angle that the spokes make in relation to the rim, the more perpendicular the sokes are to the rim, the shorter they are, and vice-versa. For strength reasons, if there are fewer spokes in a wheel, the spokes are often set at a greater angle from perpendicular, and therefore need to be longer. It only takes the spoke length being off by a few millimeters to make them not work.
I don't know if the 40-spoke versus 36-spoke Honda wheels use different angles at the rim, but when custom wheels are put together using otherwise stock pieces from different models, it is common to end up needing non-stock length spokes, which can sometimes only be determined while building the wheel, making it tough to select them from a website, or be quoted via email or over the phone. If you can talk to someone who has built the same wheels using the same hub and rim combination, you might be able to get a recommendation on spoke length, but I would be prepared to have to experiment a bit to get it right.