Combining Thunderbolt ports
Your computer has a "Thunderbolt 1" port. Apple states in this FAQ article on thunderbolt ports as an answer to question #25:


  4K Ultra HD TVs are supported over HDMI or with Thunderbolt to
  high-speed HDMI adapters. Thunderbolt 2 is required to use a 4K
  display. All Thunderbolt 2 models support 4K displays at 30Hz in
  Single Stream Transport mode. Some models support 4K displays at 60 Hz
  in Multi Stream Transport mode. See HT6008 for more information on
  using 4K displays. Specific 4K DisplayPort displays are supported on
  Thunderbolt with a mini DisplayPort to DisplayPort cable.