10.5.x has the new data engine (Hyper) and there is no tech reason as to why not to go to this and reap the benefits of the fast speed and new functionality.  In other words, you can skip versions such as 10.4

Something to maybe watch out for though is if you are running with extracts on server, the first time that they get refreshed on server, then the format will convert to Hyper.  What this implies is that for your desktop users to be able to access downloaded copies of the workbooks, they need to have the 10.5 desktop to be compatible.

Yup, I agree w/Jeff, no reason not to get the latest and greatest.