Slugs! -- ate my entire crop last summer, while I hadn't seen a single one in previous years. I'll be doing something for the slugs this spring (diatomaceous earth?)
only problem was last year the rain at the end of the summer.... boo urns
In a guerilla grow without much attention, GG will usually yield 2 to 3 ounces. However, I would definitely not say that 4 ounces is max. I've grown several 8 or 9 ouncers, even in guerilla situations and I've seen others do quite a bit better. Hey, THC123 grew one that weighed over 20 ounces, but that one stayed in veg much longer than usual due to light pollution.
If you are growing in a backyard type situation where you can provide lots of care, the average would be about 4 ounces.
Greens