Yamaha makes a digital 4 track that uses mini disks...the quality far surpasses anything you can get out of an analog 4 track using cassettes. I can say this with authority as I used a Tascam 246 for many years and the difference is like night and day especially in terms of sound and headroom...and it's very affordable....The sound is damn near CD quality. Check it out on Yamaha's website...under digital recorders..

(I use their 8 track version and love it.)