Frank,
If your Model 4 has the TANDY Power Supply with a Single row of Pins one one end
of the Power Supply, you need to remove the Supply and check the solder around each
Pin with a magnifying glass. You will find little circles where the Pin isn't soldered well
to the PCB. Re-flow the solder with Kester Rosin Core, and it will be fine.
Also if your Model 4 has Tandon TM-100 Series floppy drives, open the Floppy Door and
look at the hinge area with a Flashlight. If you see two short White Pins, they need to be
replaced with a full length Brass Rod. You might also have a Hinge part that is broken and
needs to be replaced. I've got SPARE parts that are Resin Printed with TOUGH Resin.
Let me know, and I can send you a couple kits to fix your Model 4.
Larry