Comment out some part of a line in matlab function

Nope, this is not possible. From help '%':

%   Percent.  The percent symbol is used to begin comments.
    Logically, it serves as an end-of-line character. Any
    following text on the line is ignored or printed by the
    HELP system.

So just copy-paste the line, or write a tiny function so that it's easier to switch between versions.

Within one line is not possible (afaik), but you can split up your term into multiple lines:

x=1+2+3 ... optional comments for each line
... * factorA ... can be inserted here
* factorB ...
+4;

Here * factorA is commented out and * factorB is used, resulting in the term x=1+2+3*factorB+4.

The documentation contains a similar example, commenting out one part of an array:

header = ['Last Name, ',      ...
          'First Name, ',     ...
      ... 'Middle Initial, ', ...
          'Title']