Can I still trade 4:1 with the bank if I own any 3:1 or 2:1 port(s) just to get rid of more cards?
Based on my read of the rules, specifically the section in the Almanac on Maritime Trade, I don't see any reason why you couldn't trade at the 4:1 rate even when you have a better rate available. I even see this text:


  Generic Harbor (3:1): Here you may exchange 3 identical resource cards for 1 other resource card during your trade phase.


The use of the word "may" makes me think it's optional. If they wanted you to only be able to trade 3:1 after getting a port, they probably would have used stronger language.