There is
no binary difference between a package and a port. The latter is merely built locally.
If you are running 4.5-release/4.5-stable, the version in the ports tree is ClamAV 0.94.2. If you cannot wait for 4.6, on October 1, which is synced with the newer ClamAV 0.95.2, you can move to -current, which has it.
Using a -current ports tree with a -release/-stable OS is unsupported. Before even attempting it, read FAQ 15.4.1. Then, read it again.
I recommend following the instructions in the warning you copied and pasted.