LogFAQs > #976586189

When you reach later collector levels you start getting caches that will be guaranteed in a set of 4 to have

1 new card
2 known predetermined S4/S5 cards (2 s5 if new card is s4, else 1 s4 1 s5)
1 random s4/s5 card

(example currently it is Man Thing as new s4, Lady Deathstrike and Silver Samurai as returning s5)

If the card is owned you get a special exclusive variant for it instead

But they only appear every 100 or so levels so you need to be frugal with them and time them for new releases you want. (If you use 4 on the same set you will get the new card guaranteed)