Assuming you are talking about a general public transport ticket, you can buy a 'anonieme ov-chipkaart' (anonymous public transport card). Sadly there is no such thing as a 4 day ticket, you pay as you go.
These cards are available at a variety of places. At Sloterdijk you can enter the train station to buy one from a machine (labeled with the ov-chipkaart logo) or the ticket office.
What public transport can I use with an OV Chipkaart?
You may travel by train, bus, metro and tram. Depending on the means of transport you will need a deposit + enough money to travel.
How much deposit do I need on my card?
Train: 20 euro
Bus: 4 euro
Tram: 4 euro
Metro: 4 euro
How do I put extra money on my card?
You can only load more money onto your card at a machine labeled with the OV chipkaart logo. These are often found in train stations or metro stations. You can not do this while in public transport. However, in trams it's often possible to buy 1 ticket, but this will be more expensive than paying by ov-chipkaart.
Do I have to pay for the anonymous ov-chipcard?
Yes, you do. It costs 7,50 to print the card.
I just found this, which is new to me. But this is a 3 day card for 25 euro. It gives you unlimited use of public transport in Amsterdam.
This plus some lose tickets for the last day might be the cheapest option. I won't cover other cities than Amsterdam though.