Anyone have a suggestion of a good setup? I assume some sort of surround sound or four speaker setup?

That kind of multi-speaker setup works for background music at low levels, or for true surround content ala BluRays where you have dialogue and music coming from the front speakers and sound effects coming from the back speakers. (Note that this is how movie theaters are set up...)

If you're talking about a "double stereo" setup for movies with 4 speakers, one in each corner, that's not gonna work how you want it to. What's going to happen is weird phase issues and the speakers actually canceling each other out to an extent.

but found that they were not loud enough to transmit the sound the full length of my backyard (30ft in length or so). It was either too loud for those in the front, or too quiet for those in the back.

What kind of wireless speakers we talking here?

Sounds like they were just underpowered. If it was overwhelmingly loud for the people up front it should've been audible in back. Sometimes "distortion" is mistaken for "loudness" -- I'm thinking maybe things sounded "too loud" up front because the wireless speakers were simply distorting to shit.

For comparison, I have a yard of similar dimensions and we've done outdoor movie nights with Logitech computer speakers from 2001: http://www.amazon.com/Logitech-Computer-Speakers-5-Speaker-Black/dp/B00006B9CN Those don't give movie theater-quality sound but they've been fine.

If you live anywhere near a populated area, you should be able to find a reputable home-theater receiver on Craigslist for well under $100 (probably closer to $50) that, paired with some modest wired speakers, should put out more than enough power for movie night -- especially if you add a powered sub or two.