I had the same problem but i find some way's to fix it 1.Low Temperatures, IDK why, but they affect the phone, they make the screen flick even in med bright 2.Bring bright to max 3.Charguing it, yes it sounds crazy but when i chargue it in low bright whit energy saving on, the flick stops 4. Droping it to the floor, yes it sounds crazy too, but well, it just happens xD, the phone restarted and stop flickering for the rest of the day.
well, i want the same, but dont use 4k monitor , The graphic card ( i think the gt 750m) well can support it , but things like playing videos can produce some lag, the connector are the same so no problems whit that, the fit the same, If you dont go to play intensive games do it, but pls take a photos, and upload it. I recommend to pucharse a burned imac 4k and get out the pannel and install in this. Update (09/17/2016): Another option mi friend is Pucharse a Akitio Thunder2 PcieBox ( oficially dont suport gpus but well thats bull**** because there is rivals whit high prices ( COF COF MAGMA, BIZON) but well, pucharse and install a good gtx 1060 or even 1070 ( no 1080 because the cpu go to botlleneck), pucharse a power supply of 500w and install it and enjoy it whit a 4k external monitor, and the other port a good sdd of 800mb/s of write/read would to be ok