so here's my situation currently, im trying to build a crystal oscillator circuit that would work for a 40KHZ ultrasonic sensor, model SSQC SQ-40T and SSQC SQ-40T. i've done a decent amount of research about this but i'm not sure which oscillator is most suitable for this: TTL, CMOS, or pierce oscillator? i've been looking at mostly the Pierce oscillator and how they work on this site:

Pierce-Gate Crystal Oscillator, an Introduction

what i dont understand from that site is the example they showed, which is with the equation to get all the values within the circuit. Can someone show me
how that equation works? or an equation for getting the values for any of the other oscillator cmos, ttl.. etc, and another thing is for the U1 inverter gate in that pierce oscillator, can any gate be used? (sorry i'm abit unsure about the gate so i'm just assuming)

My second problem is that it is impossible for me to find a 40KHZ crystal, however what i have found is a 4MHZ crystal:
4.000MHz Crystal - Jaycar Electronics

what i do want to know, is there any way to divide a crystal by 100 to get 40KHZ? i know about that its possbile to divide it in 2,4,8, and 16. But 100 seems a lot? so if there's any way please advise me.

please also comment whether i'm on the right track or just completely gone off the topic, i just need a lot of help about this. i'm a beginer at electronics

yeh i would of use an IR if it wasnt too late, the amplifier has already been built pretty much, im just stuck on this oscillator thats all.