As you plan to drive four speakers, have you thought about building four 250 watt amplifiers ?
Maybe build active speakers and put the amplifiers inside the speaker boxes. Then for outdoor use, you can just keep adding more active speakers.
Six would give you 1,500 watts etc...
Or stereo, its a far more flexible system for really high power, and there is some redundancy if one box spits the dummy.
That will be difficult enough, but its a much more manageable project.
If you are making it on circuit boards, it just as easy to make four as to make one.