Do you have TSupport or something similar? The first thing is if BES 5 will cost money
The single biggest things that an IT director might be interested in is roles, where a Help Desk type can do certain things, but don't have access to all the functions. Also, High Availability. You can have multiple BES servers (one license) and fail over during patches, emergency downtime etc. The secondary BES handles everything seamlessly to the users. It only needs one SRP since they don't connect at the same time.
Biggest user changes (if they have OS5 on the device), better Outlook folder management, flags sync.
Of course the biggest change overall is BAS, the web based admin. Honestly, I think I miss the old console.
Also, if you do have TSupport, keep in mind the product lifecycle. 4.1.7 is end of support July 2, 2011, which for TSupport customers is defined as:
"Customer with existing contract continues to be supported; all issues are triaged but if the issue reported has been resolved in a current version, customer may be instructed to update the software. Customer may continue to receive hot fixes for Sev 1 and security issues."
I'd probably wait until 5.0 SP2 though.