Hardware Test Error 4BAT/6/4000004: 0X89850b90

Appears to be a Battery error.

What does it say from the battery icon in the upper right menubar.

Hold the option Key and click the icon for condition.

AHT is not definitive by any means. This could be a voltage, current or temperature problem.

You can, and I would take it in for a free 'Apple Service Diagnostics' test. Present them with the error code.

In this way you have an incident report on file, if there is a problem in the future and your warranty runs out you already have started a case—this will work to your advantage to have a record of a known issue with your machine.

You may or may not be under warranty but you still get the free comprehensive diagnostic

Genius reservation http://www.apple.com/retail/geniusbar/

on-line https://getsupport.apple.com/GetproductgroupList.action

check warranty https://selfsolve.apple.com/agreementWarrantyDynamic.do