myplot = right("0000" & myplot,4) returns the correctly formatted 4-char string.
so "12" becomes "0012"
"12A" becomes "012A". If you need this to be "0012A" then it needs a bit more work.
Did you see this ? I don't have the time to fully understand your problem, but I need to convert a number to form part of a filename, and want the filenames to sort sensibly (so I will have filenames fn0001, fn0002, ... fn0009, fn0010, etc.). I use the above approach. But I don't have to parse the numbers out from a string and back in again - though that doesn't sound too onerous.