How do you transfer all data in MacPro (4 drives) to iMac

I cannot give a detailed answer because I have not experienced this process. However, my first reaction to your post was: how do expect 4 x 1 tb data to fit on the new 1 x 1 tb drive?

Additionally, I would expect it to port over from one drive you choose (but I am not certain on that). If that is correct, you would need to manually do whatever else. Also note that your older apps or files may not be compatible with the latest OS Catalina.

Migration Assistant only migrates 1 drive. Select the drive that has Mac OS, your apps and your critical data. The other drives you can copy or if they are going to separate external drives you can clone them using a cloning app like Carbon Copy Cloner or SuperDuper!.

You may want to consult with your local Apple Authorized Service Provider for additional advice.

That would work.