Although not always good advice, in this case I have to say "not
broken, don't fix it".
The standard practice is no vent. Most nearly every house in the entire
country is built this way, and not because people are cheap -- because
this is the best way to do it. I would be surprised if you can even
find a product sold anywhere meant for capping a dwv stack, so that
right there should tell you that it really is pointless and stupid.
Squirrels don't like living in shit. Neither do birds. Or even mice for
that matter. There is nothing down that pipe except a 20+ foot drop
straight down, with smooth sheer vertical walls, then its all sewer,
slime, and yuck at the bottom. A few leaves, pinecones, whatever are
not going to cause any problem at all, any more than the, um, crap, you
put down the toilet. Remember that is a 4" diameter pipe.
A screen will catch the leaves and snow, blocking your vent. A cap can
get clogged by leaves and debris, possibly, or more likely just create
a perfect, dry place for wasps and bees to nest, blocking your vent.
The case of your utility vent stack is completely different, with
different requirements (it needs to stay hot, and clean, and preferably
dry I suppose). Your sewage stack is exactly that -- a smelly, wet,
The "squirrel or bird down the vent stack" sounds about as likely to me
as a mouse wedging itself in your hose bib. Sure, it could happen, but
the once in several lifetimes event is simply not worth protecting
against. There are better things to worry about, and your just causing
yourself more trouble than you are saving. Besides, that would have to
be some lard-butt squirrel to clog a 4" pipe, no?. The ones around here
aren't that big.