Only 3 out of 4 elements heat up in my furnace and all 4 are good what's wrong ?

If all are good, then it would be wiring, connections? Try swapping a good and a bad, then see which one fails, the same Element, or the same location?

One of your units heat sequencers is probably defective no allowing the one heater to come on.

How did you test the elements, to arrive at the conclusion that they are good? A clamp-type ammeter will reveal which elements are working, and a voltmeter will reveal where power is being interrupted to any that aren't. Heat relay/sequencer, power wiring, limit control, fusible link, broken heater string, or power supply/fuse/circuit breaker are the possible culprits. You may also check the control wiring for the possibility that the relay that controls the element in question isn't even hooked up.