After a couple of times around the loop I looked properly at the warnings on that final page before the "install" button, and made some php.ini changes in line with what it requested - increasing the memory_limit and the upload_file_size. I also installed php5-curl (I'm an ubuntu user so this is just an aptitude package for me) and the install ran like a dream at that point. I'm disappointed that SugarCRM couldn't give me better feedback than just a 404, but it seems like it needed some settings that I didn't have - so if you see the same behaviour, don't give up but heed the warnings and it should be able to install itself absolutely fine. Hope this helps!
I noticed that SugarCRM have just released their new version 6.0.0, and since my sugarcrm installation is madly out of date and I'm about to start using it again, I thought I'd just throw the old one away and install from scratch. I had no problems until I reached the final installation stage, when clicking the "install" button would return a 404. This is tedious because then you have to follow the instructions and change config.php so that "installer_locked" is false (but the installer does remember all the information you give it, which makes this less annoying)