It actually seems the U10N was reported as being unable to write DVD-RAM by whoever entered it into the VideoHelp database (typically helpful in determining specs on drives). If the drive did have it enabled, it was probably 5x max write speed.
Looking at that, and comparing it to the entry for the 4083N, your current drive has a slower write speed for +RW and -RW, as well as for +R DL and -R DL.
Whether this difference in speed should concern you is something entirely different. Unless you go out of your way, you will probably only find 4x +RW or -RW at the max. We wouldn't recommend you use a slim drive to burn DL media, so the extra "slowness" there may never matter.
Using quality media, I'd say there's no reason to be concerned or upset. You're still getting a decent drive, capable of writing to RAM with the same reliability (albeit at 3x max, but that will not matter much unless your -RAM media is rated for faster than 3x & the U10N wrote faster than 3x, if at all).
I would just focus on testing the drive to ensure it meets your needs right now. If it works, don't worry about it. If it fails to equal the U10N's performance (fails to write on a favorite type of media, etc), ask HP to send another replacement of come here to try to find a fix/workaround.