The 4 business units preference is to have 4 separate instances of Sitefinity on one server/one base domain, just so they have full control of global preference settings, i.e. language support etc...
When you say in the prior post
" If you try to write to a location outside of the application's root you might have permission issues."
Do you mean the application root of sitefinity? Or the application root of the Website?
If we structure our install as described above (1 server/ subsites on same base domain/ 4 instances of Sitefinity)
If a customer is using the search engine on businessunit1.mainsite.com, will they be able to return results from the other three business unit subsites as well? (businessunit2.mainsite.com, businessunit3.mainsite.com, businessunit4.mainsite.com)
We need to be certain that this will be absolutely possible as we plan on purchasing 4 instances on Monday.
Thank you for your quick reply,