I've used PVC pipe before to make target stands. It works ok, but there are a couple drawbacks that made me stop using it. First, if you accidently shoot it, it'll shatter and destroy that entire section of pipe. If you've used PVC pipe cement to join the pipes and joints, then you're screwed because I know of no easy way to debond the joints for repairs. If you don't use PVC cement, the stand will tend to fall apart and can be a major PITA. Also, a lot of ranges will not take kindly to PVC fragments left on the range.

The second problem is that the only way to affix a target and backer is with tape or other adhesive, or by using a number of clamps. Using a staple gun, which is the fastest, easiest and my most preferred method, won't work with PVC (it'll break, see above).

Now I just use a suitably sized cardboard box with a couple thin metal posts poked through vertically to hold it to the ground. I'll also weight the box down with gravel or sandbags if necessary. But this puts the target pretty close to the ground, so it wouldn't meet the 4' high requirement (why it that requirement there anyway?).

I'd recommend to make a target stand out of the cheapest 2"x2" lumber you can get. Assemble it with carriage bolts and wingnuts on the bottom of the verticals to make it foldable or easy to disassemble for transport and storage. You can staple to wood, and the wood will generally take a few hits before falling apart.

Mike