If i try booting from floppy (tried WB2 WB3 and WB3.1) but just get "WORKBENCH SCREEN" ... no icons not even of floppy.
The A4000T needs a special Workbench3.1 disk which contains workbench.library. On other Amiga models this library is in the Kickstart ROM, but not on the A4000T.
What did you do to the partitions that it does no longer boot from HDD ?
For the initial problem, once it boots from HDD again, hold down both mouse buttons after a reset, click on "boot without startup menu" and in the shell window enter
delete prefs/env-archive/sys/screenmode.prefs
After that it will boot into the default resolution.
If you need HDToolbox to edit partitions, you can do the same (boot without startup-sequence) with the Install3.1 disk in the drive and run HDToolbox by entering the command into the shell window:
cd hdtools
hdtoolbox
Bye,
Thomas
Edit: BTW, your signature shows no less than 5 Amiga models which could be used to repair the harddrive very comfortably. Especially the A4000 is predestinated for that. Just connect the A4000T HDD instead of the DVD drive.