I used a CompServ Arxxon scandoubler successfully with various TFT monitors.
Check out your monitor controls, modify the clock and sync and you'll get rid of the vertical lines.
PS: If your monitor doesn't show anything put your A4000 jumper in NTSC mode and it will work
edit: um! of course, if you have interference in the screen you should check your miggy shielding and also use a proper vga cable (although most monitor cables included by the manofacturer are usually OK)
