You can find out their WIFI Mac address if they are on your network, but not their 4g mac address.
What is the reason for this? If someone is attacking your network from their phone you should have logs of access attempts at router to block that IP.
With that IP you should also be able to report them to the carrier (if using data and not wifi).
Is there a way to find a cell phones mac address using their phone number or I.P address?
I have already answered this.
The only way you can cross reference mac address to ip address is if the device is on your wifi network, in which case you have an ARP table, so you can cross reference the two.
If someone is using their cell on wifi to access something on the internet, then the IP address you see is not even the IP of the phone but of the router.
If this is someone using 4g you have no way to cross reference on your own.
If this is a legal matter then have police/lawyer track it through cell carrier.