Can I use dual 4k Dell P2815Q monitors on the Retina MacBook Pro (late 2013) with Yosemite at full resolution?
This model supports a simultaneous maximum resolution up to 2560x1600 on two external displays via Thunderbolt. 

Alternately, it can support a single display up to 2560x1600 via Thunderbolt and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.