The short answer is . Capcom never officially developed or released Resident Evil 4 for the PSP.
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While the game has been ported to almost every modern console—including the PlayStation 2, Nintendo Wii , and even mobile phones—the PSP hardware was generally considered too limited to handle the full, uncompromised 2005 experience. A project titled Resident Evil Portable was announced for the PSP in 2009, but it never surfaced and is widely considered cancelled. The short answer is