I will try to post all the info here, though I admit i am in a rush, posting this.
On my older desktop here, I can boot to a TTY but even with some troubleshooting I have been trying, I cant get the GUI to come back.
some general info is that the 3.99 ISO boots fine, and I have done (@ the date of the 4.0 ISO releases) alot of updates ... including kernel and xorg package updates I see and have rebooted a few times... all was fine ... I install the last few updates including lightdm (not saying that is 100% the cause) and that is when the issue started then... I have reformated and done the same thing ... with the same results. I also have to note that while the 3.99 ISO Mate initially boots fine ... the 4.0 Mate ISO had the same "only booting to a TTY" issue.