My understanding is that in the new driver design wth FreeBSD 10 they're kernel mode drivers that don't need to be compiled into the kernel.
You can build the kernel drivers by:
cd /usr/src/sys/modules/mlx4
make
make install
then do the same for ibcore, mlx4ib, ipoib and mlxen
then you can kldload them or add them to your /boot/loader.conf (mlx4_load="YES" etc..)