the relationship between the amount of mana you spend and the effectiveness of the P/T you get out of it is not linear, it's some curved thing.
for cheaper cards, the P/T output is not worth the card. for more expensive cards, the P/T output is not worth the mana (or is often too slow to be relevant).
the exact relationship depends a lot on the format. the extreme examples would be how a 1 mana vanilla 1/1 is basically not worth a card ever without some serious rules text backing it up. on the other hand, a 10 mana 10/10 would never be worth it without some serious rules text as well.
for very many formats the "best" point in that curve is the 4 mana 4/4.
1 factor of this has to do with the ability of cards to double block
for example to double block a 4/4 with some lower combination, your choices are two cards with power 1+3, 2+2, 2+3, or 3+3. on the other hand to double block a 5/5 you can use 1+4, 2+3, 2+4, 3+4, 3+3, etc.
another issue there is the cost of removal, and the fact that later in the game the opponent will have more mana to spend on clearing the path (making more expensive drops doubly vulnerable). putting more mana into 1 creature when it might be able to go somewhere else is worth less and less, because it can all be "lost" with 1 removal spell. in the same way, 1 creature can block it, and nullify its offensive power.
a 4 power creature tends to be able to remove many relevant planeswalkers in a single swing the turn they drop
another issue here is that often the cutoff for conditional removal often is toughness or cmc 3 (even in formats without bolt, we often have some variant on
volcanic hammer or
smother)
another aspect is players have 20 life.
the amount of turns it takes a creature of a given power to kill a player is:
1 power takes 20 turns
2 power takes 10 turns
3 power takes 7 turns
4 power takes 5 turns <-
5 power takes 4 turns
6 power takes 4 turns
7 power takes 3 turns
8 power takes 3 turns
9 power takes 3 turns
10 power takes 2 turns
notice that we have diminishing returns on this for the more power we put in! and 3 -> 4 power is the last jump by more than 1 turn.
also, in a lot of formats, turn 4 is basically the last turn to finally play something if you didn't play any other spells this game, and still have a chance to be in the game. (how big of a chance varies a lot by format).
i think the big notable exception to all this is vintage, which once upon a time had people playing
slash panther because the resource system does not function how you would expect. (although the 4 power to kill planeswalkers was still relevant in that decision!)