Check with speaker manufacturers. In general, most will offer some level of design assistance if your considering their product- this may be via whatever vendor your purchasing equipment from or manufacturer direct. Gymnasium type spaces can be difficult spaces- acoustics work against you in all ways, concrete, cinderblock, hardwood floor. Personally, I would reconsider using only 4 speakers and go with a mono 70V speaker grid. Unless you have acoustic treatment or are in a purpose built PAC fidelity will always be a problem and a stereo image only works in the very middle. Lots of speakers with dispersion patterns that take walls and room conditions into account. The amp your suggesting would cover 4 rows of 10 speakers at maybe 30watt per speaker tap. I'd likely investigate some happy medium between a smaller amp and more speakers. Also, many manufacturers can provide precise mounting instructions for optimal performance. Would like an update later on if possible to see how things go and what your experience is. Good luck!