If you get pro pricing is it for one project or do you have to also pay the pro pricing for every project? For example, if I have 4 free tier projects and want to start another project with pro, do I now have to pay $125 a month or can I pay $25 and only have pro features for that one project?

Projects are billed individually. Upgrading the plan for one project doesn't upgrade the others, so if you choose to only upgrade 1/3 projects, you'll only pay the plan price of 1 project (1x25 = 25) while the others will remain on the free tier. Likewise, if you upgrade 3/3 projects, you'll pay the plan price for 3 projects (e.g. 3x25 = 75)