FreeBSD crashes copying large files in VM

AFAIK bhyve does not have the possibility (like kvm or virtualbox) to cap the physical core count independent of virtual number chosen.
Perhaps you could downscale the 4core VM to 2 cores (leaving at least 2 for handling sysio) and see if this helps prevent your problem. At least this would confirm the overcommit is part of the problem or whether this is just haunting a ghost.

Can't really advise you on further tuning of bhyve - there might be a way (-p option, -H option) to forcibly hold all your VM's to a limited number of cores. Even my previous advice might be too conservative.
bhyve gurus : please step in!