The 10-4 is sort of rare. If pristine a collector trying to round out a Model 10 collection would want it. The only difference between the 10-4 and the 10-5 is a slightly thicker front sight. The 10-4 was only made during 1962 but it was a high volume production model. So even though it wasn't made for very long, they made quite a few of them.
$550? I'd pay that if I wanted to have something that not every Tom, Dick and Harry could whip out at the range. If I just wanted a Model 10 shooter I go for a $350 10-5.