Hi there Ben,
Please let me start by pointing out that we have a Nonprofit Partnership program that allows approved organizations to use Events Calendar PRO for free, as well as providing discounts on our other plugins. You can read our Nonprofit Program Knowledgebase article to get all the details. On that page you’ll find a link to our application form with further instructions.
If for some reason you are not able to apply, and to answer your question if you just plan to have 4 websites then a Business + a Personal license for the new site sounds like the best option in price. If you plan to have more than 4 sites then an Agency license would be cheaper.
Thanks and we look forward to hearing from you!
Best,
Nico