4k projected as such looks better than 2k.
4k downsampled to 2k looks better than shooting 2k native (on any camera).
4k gives you room to move around the shot with all that lovely extra resolution.
4k is more future proofed than 2k.
4k will be commonplace faster than you think.
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http://www.nattress.com - Film Effects and Standards Conversion for FCP