Cannot start server after upgrading to 4.0

I upgraded from 3.4 to 3.6 and to 4.0
The upgrade to 3.6 secceded bu when upgrading to 4.0 it fails with the following error:

I STORAGE [initandlisten] exception in initAndListen: 28574 Cannot start server. Detected data files in /var/lib/mongo created by storage engine ‘wiredTiger’. The configured storage engine is ‘mmapv1’., terminating

Hi danb@storenext it looks like a setting in your process.

It could also be that you have the MMAPv1 data located at a different path than what the data files.

Let us know if you have any other questions.