More info needed ...
New T_4.1's - soldered in any way after working to blink from new? Every programmed with anything to work?
USB Host is the internal pin array to Host USB devices - was that used and connected to something?
The connector on the end is USB Device connection. That is used for programming and powering as well and communications to Host devices.
Details on what was done and could lead to assistance.
Plugging in the end Micro USB connector to a Host computer from new will Blink when powered. That will show 5V on the VIN pin from USB cable and when powered there will be 3.3V on the 3.3V pin when measured with a meter against a GND pin on the Teensy.