Reply To: Apply style to every other/every third paragraph?

Watch it, you are using regular Find notation instead of GREP!

You could use (theoretically!) something like this: search for

^(.+r.+r.+r.+)r

and replace with

$1n

I don't think I would let such a replace go nuts on my text. You say it's a “product line”; I think I would play it safe and check what makes these a product line, rather than 'regular' text (i.e., a “Product name”, or Stuff With Capitals, or Added Serial Numbers-20B).