Early 2013 Macbook Pro with 4K
Yes, it is possible to drive a 4k Display with an early 2013 and even 2012 Retina Macbook.
1. Download SwitchResX
here
2. Create a new custom profile for your monitor
Simplified Settings: CVT-RB
Active: 3840x2160
Scan Rate vertical: 30

Hit ok and press cmd-S to save.
3. Reboot and select custom profile

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