4 nights in Krakow gives you enough time to spend a nice city break in Krakow.
With regards to your question about the lockers- yes, the main station offers lockers to rent – I don’t know where you will be staying, but majority of hotels / hostels will store your luggage for free even before the check in official time.
When it comes to the other questions, my personal top 10 would be:
- Taking a walk around Rynek Glowny and Kazimierz districts. If you’re staying within the Old Town area you can do it on your own with a good guide book. Alternatively if you have a bit of budget you can opt for a personalised tour with agencies such as e.g.: https://krakowzwiedzanie.pl/en/ or if or if you don't mind group tours you can join Free Walking Tours
2,. Take a Vistula river cruise to Tyniec Monastery you can book tickets )- you can book Tickets here https://www.e-statek.pl/
- Visiting Krakow underground museum http://podziemiarynku.com/
4.Take a guided bar crawl - Krakow has huge number of bars and clubs, and it’s worth booking a guided tour to visit. If you are a single traveller most people wouldn’t feel comfortable going to clubs on their own. But luckily there are companies such as e.g: Booze Kult Crawl - they gather backpackers from all around the otown and will take you to 4–5 venues and you will have a chance to meet people from all over the world
- Vodka tasting ! When in country of vodka’s origin it’d be a shame not to try this experience. Here is a link to a great show, and it’s not only for Krakow stag do groups – individuals are also catered for https://partykrakow.co.uk/krakow-stag-do-activities/magic-of-polish-vodka-premium-tasting-experience
6.Wieliczka Salt mine - a must do for single travellers or couples in my opinion. An organised tour with hotel pick up is usually around 40 EU
7.Auschwitz Birkenau / Oswiecim- again a must do/visit. Plenty of agents offering this tour for roughly 30–40 EU/pp
8.How about a tour to Zakopane, which is a Polish winter holiday resort located in the Tatra mountains ? It’s about 110 km from Krakow, but 2,5hrs drive on average. There are some hot springs in the area e.g.: Chochołowskie Termy - Największe Termy na Podhalu
9.Ojcow National Park- only a short drive from Krakow, with a great looking Pieskowa Skala castle
- It’s not a tour, but a great way to save money on entry fees and public transport https://www.krakowcard.com/
Hope this helps a bit