I'll go for
A divine trapped in square!
Playing cards are rectangular (close enough to square). The "divine" bit could be a reference to tarot cards.
Only silences to your prayers,
You might pray to win a game of cards...
Until a step through the stairs.
... but only luck will see your prayers answered.
Powers to make you an millionaire,
The only one with a spear.
If your luck runs good (or you cheat, or you run a casino)$^1$, you can become a millionaire by playing cards. Many games have only one winner.
Beneath the future heir,
The Jack or Knave in a given suit can sometimes be called the Prince, heir to the throne.
Four does not make 2 pairs!
Four suits in a pack of cards.
$^1$ Well, OK, the two possibilities in brackets are the only real ways of doing it, unless your name's Mat Cauthon.