SHANGHAI (Reuters) - China's yuan hit 6.4515 to the US dollar in mid-morning trade on Friday, its lowest level in nearly four and 1/2 years, raising questions over how far Beijing will let the currency weaken to help shore up economic growth.

It was also weaker than the lowest level reached in mid-August after a central bank surprised global markets with a devaluation.

Friday's weakening came after the People's Bank of China set its official midpoint rate at 6.4358 per US dollar,its weakest level since Aug 5, 2011, prior to market opening.

Last week, the International Monetary Fund announced that it would include the yuan in its Special Drawing Rights (SDR) basket, an important milestone in China's integration into global finance.

Markets have been rife with speculation that Beijing would allow the yuan to depreciate after the SDR inclusion, and the yuan's performance this week appears to justify that speculation, some traders said.

However, other traders suspect the PBOC is increasing the volatility of yuan trading, letting it depreciate before the Federal Reserve's rate decision next week, and will then guide it to appreciate afterwards.

Speculators have been burnt many times by the PBOC's temporary tolerance of sharp yuan movements in one direction, followed by a hard strike back in the opposite direction.