I just finished 3 hour mow on my 4500P with the rear discharge rough cut mower deck, which brings me up to a total of 10 hours so read my impressions based on I'm new to this tractor.
I also wanted the Kawasaki engine because I have very steep slopes on my property. The dealer heavily suggested the Kubota engine because of a slight HP advantage but I went with the Kawasaki anyway.
When the dealer delivered, he said he didn't sell many of these engines but remarked that the engine was very smooth and it was nice not having to deal with a choke.
While mowing today I finally had the courage to mow my steepest hill which is 30 degrees and in one area goes slightly steeper. I felt so secure that I was going across and not up and down when it started a light rain. Right at the very end I could feel a slight slippage in the rear but I don't think I slid more than a few inches. That is when I started mowing up and down and not across. I had the weight transfer lever engaged all of the time at the fullest extent.
Final impression. Way more power than I needed. The Kawasaki engine is very powerful, does not "hunt" or "surge". When mowing at these slopes, the engine does need to be almost at full throttle or the hydraulics whine a bit.
One observation is that the engine manual says to change oil at 8 hours but not filter, so that is what I did. Oil drain is repositioned to a nice location which I liked. However, when the oil came out it had a slight "sheen" through it like there was a suspension in it. At first I thought this was just metal left over from machining, but now my best guess is that it is part of the break-in oil that had possibly a polishing media in it. Dunno. Tried searching for that on web but came up empty. Contemplated changing oil again this time with filter but then again, don't want to second guess the design engineers. Whatever the case, I would certainly recommend following the manual and changing at the 8 hour mark.