Check out www.mdfly.com. The MOL-AU5018 model will fit your needs without an Arduino. Unfortunately, it's listed as out of stock.
The other models are controlled through a serial interface that the Arduino can talk to. The MOL-AU5121 model can have up to 199 tracks. The Arduino software would need to translate button(s) pushes to serial commands to get the job done.
For your goal of four tracks, you need just 4 buttons. For more fun, you could add a keypad and allow direct track selection.