Hi there,
Hopefully you have enough storage so you can keep things that you don't use this time, especially odd pipe fittings. When you take apart or put together glued fittings always leave about two inches on either side of pipe. That way you can cut them out and reuse them elsewhere if needed. And try not to obsess about "wasting" money. Like Dr. Dave says, it's far better and less expensive to have an overage of something than to have to start over or reorder.
As for the size of your pond and liner, try to be sure you've actually got 4 ft of depth. Our first pond I dug the deep end to 54 inches for a margin of error. However, with one side of the pond being higher and the way I put in the skimmer I only ended up with 42 inches of actual depth. I used a straight board and a line level that time and it didn't cut it. I'd recommend a tripod based laser level; you can rent one for not too much. I didn't have one until late in this expansion project and I'm still off by 2 in on the one end.
What Dave is calling a negative horizon I believe is what is recommened as building up the sides of your pond so there is a slope away from the pond on all sides.
And for the liner, what Dave says again. You're talking 10 + 4 + 4+ 1 + 1 = 20 ft. That doesn't leave you any wiggle room. If you dig it a bit deeper, say 5 ft = 10 + 5 + 5 + 1.5 + 1.5 = 23. I'd get a 25 X 25 and expand the pond a little if you don't want to waste that much. One thing is for sure, the pond you build is never big enough after it's done.
take care,
Rick,