You'll need to buy a USB or Firewire interface that will convert your analog audio output from the 4-track cassette to digital audio.
If your 4-track cassette has four outs, you may need to do the transfer 2 channels at a time. Otherwise, you can mix that down to stereo tape first, then do the transfer.
4-track ---> interface ---> usb or firewire port on pc ---> audio editing software
Some locally available brands of interfaces you can tinker with: Behringer (Electronics Depot), M-Audio (JB Music), Tascam (Audiophile)