Can I use 4k Dell P2715Q monitor with Thunderbolt 27" on the Retina MacBook Pro 13" (late 2013)?
According to Apple, the 2011 and later MacBook Pro's can support up to two Thunderbolt Displays (2560x1440)

The Late 2013 MacBook Pro will support two external displays with a resolution of 2560x1600.

A 4K resolution is 3840x2160.

So, your MacBook will be able to drive both monitors, the Thunderbolt at full capability and the Dell at reduced resolution.  It won't be able to drive any monitor at 4K