UE 4 and eGPU

How will ue behave when connecting an eGPU by means of tb3 on a Mac. Will i have the choice to select a GPU from within the application and is an external screen primary screen ?

UE, like most applications, rely on the OS for hardware communication. So your OS determines what hardware applications are allowed to use. also be sure to check your controls for your gpu settings, IE, nvida control panel will indicate if an application is using what hardware. Be sure to check your system settings as well in some cases you system will determine if all, some, or no applications are using the available gpu.

Blender will let me choose which GPUs to use even if they are not connected to a display
Hoped UE4 worked the same way.

Well Blender is an entirely different program made by entirely different people. I Would not use one program to indicate features of anther.