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.
![](https://photos.imageevent.com/imoverhere/mym101s/m1015/huge/16m1015nunamaker.jpg)
If you ever see an m101 with that kind of toning you can bet with confidence it is an m101-5.