I've been wanting to start a pond, nothing to big at first. I'm thinking about ordering a 10 ft x 10 ft EPDM liner for a 4 ft x 4 ft pond, I was wondering how deep that would be? Also whats the safest amount of fish that could be put in the pond without overstocking it??

I'm also wondering what would be more "hardy" Koi or goldies such as the Ryukin or the oranda? I have been eyeing the Ogon high quality Japanese Koi's at a green house here in town. She (the owner) recently started selling pond kits and assorted high quality Japansese Koi's (pretty price as well...lol) What type of filter would I need for a pond that size??

Okay, the width plus twice the depth plus 2 feet for the edges, and you'll figure that again for width;

(4 + 2 + 2 + 2 = 10) x (4 + 2 + 2 + 2 = 10) so the depth would be 2 feet for a 10x10 liner made into a 4x4 pond. 240 gallons

You can put one koi in this pond. Koi don't like to be alone so you wouldn't want to get a koi. The best fish is a pond comet for a colder area. If you lived in a warmer climate you may be able to get a more fancey type gold fish.

Thank you, I live in Illinois, and its pretty cold. I wont be setting up my pond until the spring. How many comets can I put in the pond? I dont want to over stock. I'm familar with tanks, this would be my first pond ever.

Well, with comets its a different story than a fancy goldfish. Although everybody talks about th 10 gl per fish rule, it doesn't apply to a comet or shubunkin. First, they get so much bigger, up to a foot, or even more than that.
Secondly, pond fish in general do have so much more water available than tank fish. If you would consider 20 gl per comet, your pond would house 12 comets. For me, 12 comets in a 240 gl pond, is overstocking it a bit. I have a 300 gl rubbermaid tub, and had 7 comets in there, until they went all crazy and spawned like the dickens.

I would probably start out with 4-5 comets, and see how they handle the colder winter up there. Of course, most of them spawn naturally in the spring, so by the year after you might end up with way more fish than you bargained for...

If you want something a little fancier than a comet the wakin would also make a good pond fish. There just and hardy but a little harder to find.