A small platform extending into a small farm pond is listing badly. It
is supported on either side by a pressure-treated 4x4. Does one just
'pile-drive' a new 4x4 alongside or is there some technique involved
that is not obvious to a pale urban refugee?

You may find this helpful --until someone answers:

Dealing With Dock Rot
http://www.storeybooks.com/main/ALL_EXCERPTS/098-6/098-6excerpt.html


--Cynthia

If the pier has started listing this probably means that the pier was
not placed on a foundation.

In which case you have a couple of options. 
First is to simply drive another pressure-treated 4x4 alongside the
original leaving enough extra height to compensate for the list.
Then decouple the original from the pier.
Trim the new timber to the appropriate height and attach to the pier.

The above will only buy you a few years worth of service before having
to do it all again. The reason being is the pier supports will
eventually pack the ground beneath them and the pier will sag again.

Another answer would be to remove the pier completely and replace with
a pier with proper foundations.

Perhaps a better solution for a small pond would be a floating dock. I
have found a site http://www.canadianfishing.com/dock/index.htm which
contains instructions for building a 16x16 floating dock that should
not take much more than a week to build.

What an excellent suggestion! We can't believe we didn't think of
that. Thank you. I can't seem to find a way to route the small fee to
you as you posted a 'comment' rather than an 'answer'.

Unfortunately magicus is not a researcher (name appears in black,
researchers are blue) so you will not be able to pay for his excellent
comment.

My resident engineer suggests the following solution:

"Without knowing exactly how your platform is constructed ( or the
composition of the bottom of your pond ! ) this ought to work.

Assuming that you have crossheads bolted to the four by four piles and
these support either decking directly or girders which then support
the decking, you could take off some decking to reveal the head of the
4*4 pile and the crossheads. Then drive another 4*4 pile between the
crossheads and close to the existing pile. When this is in, bolt to it
a short length of 4*4 or 2*4 in such a position under the crossheads
that you can stand a jack on it and on top of the jack a short piece
of 4*4 which will bear up underneath the crossheads. If things are
really slack you can start jacking under the crossheads without
disconnecting them from the older pile, and this may pull the
crossheads into position and pull the pile up with it on the sagging
side. If it doesn't, then unbolt the crossheads from the pile and jack
them up with girders and some decking still on top. Once level bolt
the crossheads to the new pile and if there is enough of the old pile
sticking up, you may be able to bolt them to that too in a higher
position.

In case you are unfamiliar with the term "crossheads" these are the
pieces of timber bolted, usually to both sides of a pair of piles. "