I waited a long time to get the Police 4 and I have been totally enjoying it for the last month or so. I really like the new blade shape, it makes a big difference when cutting to have that extra belly, much more of the blade edge is usable for each cut.
My initial impression when I removed it from the box was that the new version somehow had a bigger appearance about it, and despite having the same blade and overall length somehow seemed to have different proportions than what I was accustomed to with the Police 3 that I have carried for many years. I think it is an illusion caused by the lack of blade swedge and larger opening hole on the P4. The new blade shape is definitely more Military'ish, which I really like. The handle shape is very similar to the Police 3, but with a slightly faded or washed-out outline in the grip area. At first I did not like the newish handle shape of the P4, but as much as I visually still prefer the appearance of the P3 handle, I do consider the P4 to be more comfortable in use.
Like other impression I have read about, I did find the edge sharpened with a much steeper angle than I am accustomed to getting on a Spyderco. It was still sharp, so no complaint but I always re-profile my pocket knives with my preferred edge anyway. Usually I wait to dull the original factory edge before re-profiling it but I went and did this one right away. I re-profiled it (at ~15 degrees per side) so that I can touch up my edge with a micro-bevel on the Sharpmaker with literally a couple light passes when necessary. I haven't had to touch it up yet, I would have had to touch up my VG-10 P3 once by now for my type of use - light daily tasks and food prep.
I do find myself thinking about care and maintenance of the blade much more due to the steel type, so as much as I am really happy with K390 performance, I am looking forward to trying other versions with stainless steel.
I do find the steel much less prone to rolling than VG-10 in use and the edge requires much less touch up. The K390 is a noticeably harder steel compared to VG-10. I haven't experience any chipped edge yet.
I am really happy to have what I consider to be a back lock version of the Military. I only like back lock pocket knives, but have had the Miltary for many years because it is such a nice design, despite its locking system. For the first time since I got it, I feel that my Military (CF BG-42) is redundant to me now.
I found that the fit and finish on the knife is very high, the action is very smooth even without break-in and I think that I prefer the lack of the Boyde detent (for cosmetic reasons).
Overall, I am really happy with this knife plan on keeping it permanently. Just like all previous version of the Police, for a large knife, it pockets very easily due to its slimness. The K390 steel has been a very interesting experiment for me, I never usually consider non-stainless steel and I am enjoying the properties of this steel so far.