Go ahead and upgrade the seat. Be comfortable.
Disc brakes would require different hubs, and therefore, wheels. You could probably do this for under $300 if you tried.
But a $300 upgrade on a new MSRP $689.99 bike?
Are you dissatisfied with the V-brakes in some way?
Disc brakes are not always better.
If money's burning a hole in your pocket, consider clipless pedals/shoes (if you haven't already).
That would be an ideal upgrade for the 4500.