It might have. Did you have MB and WD 3.8.3 running at the same time before?
The way WD is programmed is that it is supposed to disable itself when it sees another anti-malware solution is installed. This is because (particularly in the past), running with two anti-malware solutions at the same time was thought to cause conflicts. I liked to say it was like two dogs guarding the same bone, each wondering what the other was up to. In some cases, one program would report the other as behaving suspiciously. This is because detecting "suspicious behavior" is something these security programs also do.
Conflicts today are pretty rare, but there are cases where running two anti-malware solutions simply wastes system resources. So disabling one or the other may be necessary, or just something the user might want to do.
So to keep people from complaining that Microsoft is trying to rule the world, they coded WD to automatically step out of the way when another program like Norton, McAfee, Avast or Malwarebytes is installed.
If you did not have a previous version of MB installed before, or you uninstalled MB before upgrading to 4.0, then, as SleepDude notes, WD should have been enabled. If you did have both running before, then likely 4.0 registered itself and and disabled WD. Not sure why if you had both configured to run before as my 4.0 upgrade remembered my previous settings and did not disable WD.
Hmmm, both SleepDude and plodr have posted while I was typing above - so some duplicate information is in here.
I have both MB and Windows Defender. Do I need both?
It depends on your computing habits but probably you don't. HOWEVER, Malwarebytes and Windows Defender play very well together, and don't hog resources. I have both running at the same time on several systems here with no problems. So I recommend you do too.
Simply go in the Malwarebytes control panel, click on the gear icon in the upper right, click on the Security tab, then scroll down to the "Windows Security Center" section. Then make sure the slider for "Always register Malwarebytes in the Windows Security Center" is set to off.