bkm , It is possible that it is the switch assy. if all else looks good. Info. from ifixit trouble shooting Dremel 4000 1st link below. If the tool will not power on, and the switch lever and power cord have been checked for integrity, then the electronic assembly must be replaced. While there are multiple parts within the electronic assembly, the entire assembly is delivered as one piece and must be replaced in its entirety. Below find a disassembly guide, 2nd link below, for your dremel which will get you inside the device to check/test/verify if wires/switch/components on the board are loose/burnt/damaged. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
[[Dremel 4000 Troubleshooting]]
[guide|61106]