Due to IMEI regulations, it’s not possible for Fairphone to sell user-replaceable mother-/mainboards. If the fault in the mainboard is covered by 5 year warranty though, the mainboard replacement should be free.
Only way to get a new one I know of if yours is broken is and the problem is not covered by warranty is to send your phone to the repair center.
Edit: An official Fairphone 4 FAQ article sites the reasons as follows:
"The Fairphone 4 core module is not available for sale due to the legal and logistical constraints regarding the administration of the IMEI numbers.
If you have a broken core module, please let our Customer Support team know. We will arrange to send your device to our Service center to replace the core module safely and responsibly."