re: If you could visit any major US city for 4 days which would it be?

4 days around the beginning of November? I'd say Miami, San Diego, NYC, DC, or San Fran.

Go to DC.
Spend one day in Annapolis, one of the coolest towns in the country, great water, very walkable, Naval Academy and tons of history.
Spend another day at the Inner Harbor in Baltimore, check out Ft Mchenry, eat crabs, some cool neighborhoods there too.
And then you get two days DC

I'd probably fly into BWI, very easy and about 20 mins from all three cities. And Southwest flies there direct from MSY

I would go to Portland. I live in NYC so no need to visit.

Seattle