Anyhow, as always this maintenance release brings few new improvements rather than features such as ...
*. Few disappearing widgets GUI fixed in KDE.
*. Fixed a Kernel related bug which wouldn't let restoring VM.
*. ACPI related enhancements.
*. Screen corruption related bug fix.
*. Few Memory leaks fixed while using in GNU/Linux.
*. Solaris Guest OS installation related bug fix... these are the main ones but you can read the full details from this change-log page.
When it comes to installing virtualbox 4.0.10 in Ubuntu 11.04/10.10, etc there are few methods that you can follow. But the easiest method is to download the pre-compiled (.deb files) binaries from the Virtualbox download page which include binaries for a lot of other GNU/Linux distributions.
Just download the below packages according to your Ubuntu version (if you use older Ubuntu version then follow the above link to get it) and once the downloading is complete, just double click on the file which should start the Ubuntu Software Center and follow the on screen instructions. Good luck.
for Ubuntu 11.04 32bit or 64bit
for Ubuntu 10.10 32bit or 64bit