Hi, I am currently trying to design a low pass, 4th order butterworth filter with a -3dB frequency of around 4kHz. The overall circuit is closely based on this open source design...
http://microfluidics.utoronto.ca/gitlab/dstat/dstat-hardwareHere is an image their 4th Order Butterworth filter section:

To get the values I chose to use the Analog Devices filter wizard available here:
https://www.analog.com/designtools/en/filterwizard/And this is the circuit design and values it gave me:

I have a question regarding the 1kΩ resistors connected to the inverting input of both opamps used in the open source design, that are not present in the Analog Devices design, from my understanding the input impedance of an opamp is already very large so wouldn't these 1kΩ resistors be insignificant?
Also, I would like to do the circuit analysis of the 4th order butterworth filter to get the frequency response and confirm that the values provided by Analog Devices filter wizard do work, am I correct in my method that I could convert the systems to their Leplacian equivalents and then treat each 2nd Order Filter as a seperate system, work out thier transfer functions, and then multiply those two 2nd order transfer functions together to get the overall 4th order equation as a function of the complex variable 's'. Then replace 's' with 'jω' to get the frequency response?
Thank you in advance!