short circuit?post some quality pictures of your connections
"Hot diode", in this instance, indicates a short circuit or similar mis-wiring.Like it says in my signature - "When all else fails, check your wiring"."Eyeball" everything super-critically (solder blobs, stray strands). A) Try disconnecting everything and re-connect each section till the fault returns orB) Try disconnecting each section, one at a time, till the problem goes away. (Less preferred.)
"a 1A diode will indeed get hot if you try to push 4A through it!"If all he has is a "led strip" connected to a PS, as the OP has stated, then current ought to be minimal (not 1A, let alone 4A). Agree or Disagree?
So with the strip disconnected from the power supply (enitrely) you can use your multimeter's ohmmeter function.What is the resistance from +V to GND (red probe on +V, black probe on Gnd)?Maybe you should show a picture or two of some of the corners.If there is a problem then you have to hunt it out (no matter how big a pain that might be), specifically Method A or Method B (previous).
Between the Arduino data pin and the first LED in the string, I believe there is supposed to be a resistor (if you haven't done that). Going without can result problems.