Depends really on your audio interface as audio cards have analog to digital converters generally on the PCI card or sometimes in a breakout box unless of course your audio interface is USB or Firewire variety.

All you need to do is connect direct tape outputs from the 4 track to your PC audio interface (if in fact this tape machine has individual tape outputs for each channel). Ideally you'd want to setup some sort of syn between PC and 4 Track (likely to be MIDI for a 4 track tape based machine) unless you have enough inputs on your audio interface to transfer all 4 tracks simultaneously.