Vim multiple substitutions: three or more

As you might already know :g/#/ runs a command for all the lines containig a #, which are the topics of the different challenges.

Now, as your 1st line has deleted the "wrong" line, the 2nd just copyies the remainding one

You are on the #-Line, you move one line ahead (+1) and copy (t) it to the current line (.)

Which leaves 2 identical lines.

Building on the :s/pattern/replacement/gc idea from Samus_ (which seems to be the simplest way to ensure correct operation when pattern is contained within the replacement string), to replace the 2nd through 4th occurrences on a single line:

:call feedkeys("nyyyq") | s/pat/string/gc

feedkeys() is a function that stuffs the input string into the keyboard input queue. The point is to do the counting upfront so you don't have to worry about losing count or getting interrupted.

For a more general case, to replace the Mth through Nth occurrences on a single line for N greater than or equal to a very large M:

:call feedkeys(repeat("n", M-1) . repeat("y", N-M+1) . "q") | s/pat/string/gc

Replace M and N with the values you want (you can even let vim do the trivial mental arithmetic if you don't want to do it yourself). Note that . is VimL's string concatenation operator. Obviously this only saves keystrokes for large M. If you use this functionality frequently, it may save you time to put the above in a custom command or function of some sort, as it is quite a bit to type.