Where do you recommend I add it?
If it were me it would be Kraków as there are lots of things to do and see there. I have been a few times now and I always seem to find something new to see every time. The salt mines are definitely worth a visit, and Nowa Huta is interesting if you are into history:
en.wikipedia.org/wiki/Nowa_Huta
It's about a half hour tram ride from Kraków centre.
Possibly I might consider an extra day in Gdańsk if I were going to see Malbork castle ( about an hour away by train and well worth a visit ), as that will leave you with only 2 days and I presume you would want to go to Sopot and Gydnia while you are there.
en.wikipedia.org/wiki/Malbork_Castle
If you want to know what else to see and do in each city, search the forum, there are tons of threads with lots of helpful information.
I'm sure you will have a great time In Poland, enjoy your holiday!