He said 4.6 KHz, so by Nyquist theory double the sample rate to 10 KHz to detect a blade. To ensure a measurement at both the leading and trailing edges of the blades (and points between) double it again to 20 KHz. To measure points across the width of the blade, double it again?
Keyence
http://www.keyence.com has a line of laser sensors "LK-G/LJ-G Series" with sampling rates up to 50 KHz and a number of measurement ranges. They also have laptop-based software ... a standalone solution would include a LK-G Sensor head, LK-G controller, and LK-Navigator software.
Why not contact them at 1-888-KEYENCE and discuss your application with a sales engineer?