Set default TTL in Varnish 4.0

I am new to Varnish, and I am running v4.0 on Debian Wheezy.

I would like to set a default TTL across my cache of 4 weeks (very static content).

From reading the docs I think the answer is to set a default_ttl option somewhere in my VCL file. I have searched the docs but can only find one reference to it.

I've found this question but I think the answer must be out of date, because it doesn't work for me.

Could someone clarify how to do this in Varnish 4.0?

UPDATE: Here's my config file (the default that ships with Varnish 4.0, except that I've pointed the backend at localhost):

backend default {
    .host = "127.0.0.1";
    .port = "8080";
}

sub vcl_backend_fetch {
    set obj.ttl = 4w;
}

sub vcl_recv {
    # Happens before we check if we have this in cache already.
    #
    # Typically you clean up the request here, removing cookies you don't need,
    # rewriting the request, etc.
}

sub vcl_backend_response {
    # Happens after we have read the response headers from the backend.
    #
    # Here you clean the response headers, removing silly Set-Cookie headers
    # and other mistakes your backend does.
}

sub vcl_deliver {
    # Happens when we have all the pieces we need, and are about to send the
    # response to the client.
    #
    # You can do accounting or modifying the final object here.
}