Is a smoothie 4xc overkill for a k40 or will a 3xc work?
A 3XC is enough but may be hard to find. Seems like the 4 & 5s are the ones that are available now. The 4 or 5 will work, just has extra control capacity (for additional stepper motors, etc).
The 4 has the added benefit of the Ethernet being onboard.