Haven't tried ext4. Guess no great need but I'm glad that many are testing it.
Sounds like you might have a CD writer that is dying.
It's a pain that they aren't as long lasting as one might expect.
I finally changed one only a year old that had been giving problems writing.
Similar to yours. Second one in many years, but it does happen.
I have also had problems downloading via yakuake terminal.
Strange, and maybe it was only me. Must have been something in the timing?
At any rate, I've had good luck with the following procedure for downloading isos:
1. cd /to where you want to download it
2. $ wget -rc http://xxxxxxxxxxxxxxxxxxx.iso
[Edit: and check the md5sum]
3. Then burn it DAO/SAO at the slowest speed possible for the media you are using.