You're absolutely right! But the mere existence of the 8 track model is what makes it questionable whether $500 is a good deal for the 4 track. Otherwise I'd say without a doubt. Go for it! But all I have to go by is my own experience, and what happened to me, was I outgrew a four track machine within a year, because I wanted to add more parts (vocal harmonies etc.), but still maintain control over each instrument/vocal part at mixdown.
So if you think you could be faced with selling the machine in a year because you want something more versatile, then it's not such a great deal. If it will meet your needs for at least a few years, then it's OK. But before you lay out 500 hard earned dollars, check on e-bay to see what the md-4 and md-8 have been selling for lately. It only takes a minute, and could save you some money. Also, give some thought to what your long term goal for the machine is.
Sorry to be so vague the first time, and thanks for the wake up call i...Sites.
Twist