Replace Late 2013 iMac LCD panel with a 4k one

Hello,

I have a 21.5'' Late 2013 iMac Model A1418 (EMC 2742).

Is too %#*@ slow because of the 5400RPM HDD in order to use it for daily computing so I'm only using it as a monitor for my Late 2013 rMBP (target display mode).

I really want a 4k display in my household and I'm seeing two options:

1. buy another 4k monitor

2. mod my iMac to add 4k screen

I am aware that I cannot just replace the LCD with the 4k one because the 4k one has thicker cables which don't fit my iMac motherboard connector.

Yer I'm thinking it should be possible to add a 4k display to my Mac using a different approach: convert one of the external thunderbolt ports to a MiniDP port input connected directly to the LCD eDP connector.

If I want to use my iMac as a computer I then just add a thunderbolt cable from the seconds TB port to my modded port with DP input.

If I want to use it as a monitor I just connect an external PC or whatever to the mDP input.

I really need some technical advise from someone to make sure my idea would work.

Could someone please help?

Thanks