Hi Nathan and
to the forum.
If my telepathy is working correctly I suspect that your MP4 controller is set to 48V, but the battery you are using is only 36V
(or possibly 24V), and you cannot reprogram the controller to the correct voltage because you have the wrong USB lead.
The MP4 controller requires the latest USB lead with the 5 pin connector:
And I suspect that you may have the earlier MPIII programming lead with the 10 pin connector:
The incorrect voltage setting causes the error "4" message to be displayed on the BAC-601 Smart LCD Display, which is what I presume you are referring to as "the computer".
The incorrect voltage setting also causes the single battery bar to flash because the display unit associates the lower than expected battery voltage with a 48V battery that is completely empty.
If my suspicions are wrong regarding the USB lead
(and you do have the correct lead with a 5 pin plug) I suggest that you read
this post and then check that the "USB_SERIAL CH340" device is showing correctly in the device manager and that a COM port has actually been assigned to it.
Unfortunately I am not able to use my telepathy to determine whether all three battery monitor LEDs were lit up on your throttle unit, so it might be worth checking this to make sure the battery is showing full.
Please let us know whether or not my suspicions are correct.
Alan