DVD Burning

I have Nero 8. I edited a video (file size 4gb). I tried to burn it onto a DVD & the burning failed. I wasted a DVD. I reduced the size of the video file to 3.7gb & the burning was successful.
Questions: 1) How do I know how much video can I burn so that I will not waste a DVD disc? In my Nero screen, on the video size scale, it shows a red marker at 4.3gb so as not to exceed that which I did not but it failed to burn.
2) I checked in my Nero Burning Rom settings - Recorder - my Record of ATAPI etc. - the option is ticked where it says ‘‘DVD high compatibility mode (at least 1GB will be written’)’’. Is this the reason for it??

I do not know Nero8, I use 7. It has an option “Recode DVD-video” to burn DVD. That one will adjust the size automatically to fit DVD5 or DVD9 as you choose in selection when importing files.
I would not use it for DVD9, but DVD5 works fine and DVD play little brighter, than burned with other software.

If you wasted a DVDR recording a 4 GB video, but succeeded at 3.7 GB, that was just by chance. Size has nothing to do, the application should have warned you in case size was not going to fit into the target. 4 GB is fine.
More than 4.1 or 4.2 is risky because you are approaching the edge, where most manufacturing and handling defects happen (remember a single layer DVD holds up to 4.37 GB, NOT 4.7 GB, as 1 GB = 1,073,741,824 bytes, in common computer usage).