I would be surprised if it was only 4 mph out. BMW designs speedometers to read optimistic by about 4% + a few mph, so that they will never be liable for a speedometer reading too low.

Edit: EC Regulation 39 (speedometer accuracy) states that a speedometer may never read low, and that the indicated speed must be no more than (true speed x 1.1) + 4 km/hr. Due to production tolerances, tire size variances, etc, manufacturers will use a portion of that tolerance to ensure they never read low. BMW speedometers have long been considered some of the least accurate out there, by design.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue