When I come back like Jordan, wearing the 4-5
It ain't to play games with you, it's to aim at you, probably maim you
If I owe you I'm blowin' you to smithereens

from Jay Z – Encore Lyrics on Rap Genius

Meaning

Jay-Z’s fellow Brooklyn native Michael Jordan briefly came out of retirement after abruptly retiring in response to his father’s death, wearing the number 45 in place of his original, trademark 23™

The Black Album — on which this song appeared — was billed as Jay-Z’s “retirement album”, although hints like this make it fairly obvious that Jay was fibbing all along

“45” also refers to a Colt 45 pistol — Jay will come back “packing heat” and won’t be scared to aim a gun at you and pull the trigger, either maiming or killing you.

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