if you want a pre-built binary you can find them here: https://www.virtualbox.org/wiki/Download_Old_Builds_4_1
on this option:
Ubuntu 12.04 ("Precise Pangolin") i386 | AMD64
select i386 if you have a 32-bit PC and AMD64 for a 64-bit
but i personally would compile it from source and you can find the sources
here:
download.virtualbox.org/virtualbox/4.1.40/VirtualBox-4.1.40.tar.bz2
