Here is what I found for the 24 cards in Series #4 so far. It looks like the first print run has no letter. Those can be found printed either in Japan or Italy. In addition, I am finding print run "A" and print run "B". So I'm working on getting all four versions for Series #4.
I have every card from print run 'B' and every card but not many cards from print run "A". For the first print run (blank instead of a letter) I am missing #12 and #22 from Italy and I have all of the ones from Japan. I only have six cards from deck "A".
Here are the back variations I've seen. I have yet to check for variations on the front of the cards.
4-10 Earl Anthony - 'A' and 'B' have added awards and records
4-14 Dave Cowens - 'B' has a different caption. I don't have 'A'
4-15 Rick Barry - 'B' has different caption and awards. I don't have 'A'
4-19 Volleyball - An American Invention - 'A' and 'B' have different text
4-20 Jonty Skinner - 'B' has added awards. I don't have 'A'
4-22 Johnny Bench - 'B' has a different caption. I don't have 'A'
Jonty Skinner was the first person to break 50 seconds in the 100-m Freestyle. I worked with him in Colorado many years ago.
Card 4-24 features Bruce Jenner. The subtitle is "Mr. Track and Field". Wow!