Applying different transformations to every n'th node

I realise that something very similar has come up recently but I'm still
having problems, so my apologies if this bores you :-)

I'm using Xalan and XSLT to output as HTML.

I have some xml that goes something like:

...
  <categories>
    <category id="something">Name of Category</category>
    <category id="something">Name of Category</category>
    <category id="something">Name of Category</category>
    ...
  </categories>
...

What I want to be able to do is apply one template to every fourth category
and a different template to anything else.

Basically, what I am trying to do is create a table with 3 columns, so I
want every fourth column to start a new table row.

Insights greatly appreciated, thanks.

--
Philip McAllister
Web Developer, ticketingsolutions
pem@xxxxxxxxxxxxxxxxxxxxxx


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