We have a PS4 app, it's just not launched in the US yet due to the administrative and policy distance between SCEE and SCEA. It will be coming soon, but in the meantime, if you can get yourself a UK or EU-based PSN account, you can download the app through that and it will work fine anywhere in the world (provided you purchase a subscription through the web, the PSN store integration is a bit flaky and I don't want to promise that would work from US shores).