Re: Which Kestrel
The primary difference between the 4000 and the 4500 is that the 4500 has "Additional measurements including didgital [sic] compass, wind direction, crosswind and head/tailwind," according to KestrelMeters.com.
I don't find those additional features useful.
IMO, the best use of the wind feature of the Kestrel is to learn to judge with wind without it, i.e., when you're not shooting, estimate the wind strength, then measure it with the Kestrel. Repeat until you can get pretty close to what the Kestrel tells you.
When you're actually shooting, you rarely have time to sit and watch the Kestrel. If you do have time, by the time you have measured the wind with it, and have gotten on the rifle to shoot, the wind has changed.
So, I love my 4000, and have no plans to upgrade it.
That's my two cents, worth, probably just about that...