Okay, I will try to explain this, I hope I´m right. Lets say that player "A" is primary and player "B" is sub...
"Let Coach decide" i don´t know exactly, but i think that coach can pick also players from reserves (3rd) more often - not only afret injury of primary or secondary player, but I thing that the system of substitutions is very similar to "Coach pics from depth chart"...
"Coach pics from depth chart" means that your coach will substitute players when player A is so tired that player B will play better. Then "B" will get tired and "A" will substitute him...
"Strictly follow depth chart" order your coach to substitute player "A" only when he is totaly wasted and can´t play any more minutes, even if that means, that a few last minutes before sub he can play worst than player "B". Immediately when player "A" is rested, he goes to play and subs "B"...
"Depth chart until 4th" means that your coach will play "Strictly follow..." until 4th quarter and in 4th qarter he will play "Coach pics...". Kind of conbination....
I can say that my experience is that the more strictly you order your coach to follow your depth chart, the more will play player "A" and when you let him to decide instead of you, player "B" will play equally as player "A". I warn you that STAMINA influent this the most!!! A player with high stamina will play more likely major part of minutes!!!!