I am looking for a general technique here to apply in various ways in various situations, so please just consider these hypothetical examples at face value for discussion purposes...

Consider a number calc field called "Total" that evaluates to "4".

In designing a layout, I can use number formatting to make this field display as "4.00".

However, if I copy and paste the contents of this field, only "4" (not "4.00") winds up on the clipboard, or if try to move the contents of "Total" to a text field using a script like:

[set Field: TextField -> GetAsText(Total) & " hours"]

...the resulting data in TextField is "4 hours", not "4.00 hours".

This makes sense as to why FMP behaves like this, but I am wondering if there is an easy way to preserve custom number (and date) formatting when moving the contents of number (and date) fields into text fields.

rrr

Because 4.00 is a field proporties in designing not in file structure. Filemaker is not as ACCESS or SQL. You can create calc field which realize your wish.