Unlike the cards released later by Wizards of the Coast (which became the official Pokmon TCG), the Topsun cards were created primarily as a collectible item tied to a Japanese candy promotion
However, you can consider playing fewer than four copies of a card if: It gets worse in multiples (such as legendary creatures) It's only useful against certain matchups It's a win condition, and your control deck will have time to dig through the deck and find it You're stuck choosing between two cards which have equally strong, similar, but slightly different effects (you could play two of each) It's a slightly worse version of a card you have four copies of already, and you're including it because you want even more of that effect
For Warlock Class, the second Class ability is a one-time triggered ability that rewards your accomplishment with a card
Of those, 2 of the commons have a lurking evil version that will show up 1 in 4 times you see that common
Rootwire Amalgam In a reverse Matryoshka doll situation, a 2/3 two-drop can become a 6/6, or a five-mana 5/5 cracks open to reveal a 15/15