My '92 EX seems to switch over at or right above 4k. It's real easy to tell if you short shift to 2nd and then go full throttle from around 2k on up.
Personally, I wouldn't want it to engage any higher on my car because it doesn't have much below 4k.

Keep in mind that the car's redline and the spacing of the gears also factors into where it's set. If you shift into 2nd at the redline and the revs drop to around 4500 (like on my car) then you wouldn't want the VTEC to wait until 5200 to engage.