The differences are subtle and often very difficult to note. Fortunately, collectors do not seem to mind all that much, although I have seen some Ruth rookie card collectors get pretty fired up about it.
Generally speaking, the m101-5s are a little creamier in color and the m101-4 more white, especially on the back. I believe the stock used was just slightly different for the two sets, or at least the initial printing of m101-5. Here is another m101-5 Saier (SGC dated it wrong):
Here's an m101-4 Saier (not mine) that graded 6.5
Toning is also much more noticeable on the m101-5s, as seen by this Nunamaker, which I show as an example although the card numbers for him in the two sets are different which gives it away.
If you ever see an m101 with that kind of toning you can bet with confidence it is an m101-5.