There are very many different types of piezo devices. Depending on what it is, it could resonate from a few Hz to many MHz. Can you give us any information about it? Without that I can't offer any helpful suggestions.
Regards,
Keith

Yes my resonance frequency is 4 MHZ & I give 90 v dc square wave to TX & receive through water

 

Is your piezo designed to be used in water?
Does its datasheet say its impedance?

 

Yes my resonance frequency is 4 MHZ & I give 90 v dc square wave to TX & receive through water

If its resonant frequency is 4MHz, why are you trying to find its resonance with a 4 to 40 KHZ signal?
What are you planning on using it for; communications or imaging?

 

Most of them work at 2.4MHz and are used to make water mist, not for receiving communications.

 

Impedance ought be function of work load, means power. Thus, may calculate the power needed to excite the given amplitude in given viscousity liquid. If somehow tu acknow how many volts are giving the demanded amplitutude, then wattage will give out amperage and it , consequently, the Ohms. Other, seems wiser, method - just set the demanded power and measure volts and amperes in environment of target. However, measuring it wirh diminished power means less loss, and it may be racalced to the higher power impedance only in the case when loss are linear to power. Viscous media it shouldnt be, however - under the cavitation probably some linearity still may exist.

 

PS: actually as soon as above the 50 kHz and up to the several GHz You may build a small-power response graph by cheap thing like Nano-VNA. Cheapest are about 20 $ and most expensive about 60 $. For as an example https://www.ebay.co.uk/itm/164096187673?hash=item2634e51d19:g:7pQAAOSwSwdeVMnH