Add the water, pasta, salt and butter to the Instant Pot and give everything a gentle stir.
Set the Instant Pot to 'Manual' or 'Pressure Cook', 'High Pressure' for 2 minutes.
While the pasta cooks, shred the four cheeses.
Once cooking has completed, allow for a 2 minute NPR (Natural Pressure Release). This will let the pasta continue cooking while any foam that built up inside the pot settles. This helps prevent a mess when releasing the pressure.
After the 2 minute NPR has completed, do a slow and controlled pressure release. This can be done by giving the pressure release valve a half turn instead of opening it up all the way. Once the pressure has been released and the float valve has dropped, carefully open your pot and add the remaining ingredients. Alternatively, you can use my Flip-The-Trivet for Mess-Free Pasta Method.
Add the evaporated milk to the pasta and mix.
Add the ground mustard and shredded cheeses. Mix all the ingredients into the pasta until all the cheese melts.