Round off end of port?

[dB-SPL 10+ year member]

how large of a 45 should i be looking at adding? I'm looking at around a 4" wide port. Should it be as small as possible as to not take away from the port volume? Also, should I extend the port to factor this in?

The point of the 45 is to keep the same distance throughout the port. So measure 4" from the inside corner of the port and that's where the 45 should land.