Hi Nostromo;
First off, try googling 'PC floppy on Amiga' or something similar. I am going by memory here, be sure to get a second opinion

When you say that your 4000T's floppy says there is a bad disk, when there is nothing in the drive.... makes me wonder if the microswitches in the drive need to be cleaned. I had a 1010 drive do the same thing. You may need to take the drive apart to do this. You'll need to clean the 2 switches, using contact cleaner... Make sure it won't affect plastic or other materials.
As for using a PC drive on the Amiga... Well there's some info out there.. but you need either a modified drive, or cable.... I have a modified cable in my 1200T, seems to work 100%... ( I'm playing Blob from floppy now for 'testing' purposes

) maybe others can give more info... it's past my bedtime ......