Everything in Switzerland is closed on sunday, only large train stations and airports, as well as petrol station shops are allowed to open (plus museums, restaurants, cinemas etc of course - this isn't China or Singapore here )

Still, given the area the 4P is mostly for shopping and business, Sundays are extra slow there. However, on a nice day, it's not too far to the lake (about 10 minutes by foot) where it will be quite crowded..

Generally, the 4P is NOT at the best location if you're only doing an overnighter in Zurich. Choose a hotel near the airport, much more convenient (you can still go to Zurich downtown then from there, but as the 4P is on the other side of Zurich than the airport, just for an overnighter it's definitely not too convenient to use the 4P - different matter if you stay longer in Zurich..)