Have you checked wheter' assimps dependencies still are happily building?
You can get them by for example awk -F '|' '/assimp-4/{print $2}' /usr/ports/INDEX* or looking at http://www.freshports.org/multimedia/assimp/

"you never know"

In D12947#268839, @tcberner wrote:

Have you checked wheter' assimps dependencies still are happily building?

Yes, they build ok.