There's something wrong on your end then. When TomsHardware tested the RX 570 4GB in a system with a 7700K, a CPU not that much faster than you have, they were getting 89FPS average, and 71 FPS with a 1050Ti. When Techspot tested GTA-V with a Titan to find out where the CPU bottleneck is, it was a i7-4690, a CPU slower than what you have.
Since the numbers are higher on both, it's a problem on your end. You may need a fresh installation of Windows 10, a clean install of the drivers using DDU, or a little system cleaning to ensure your CPU is actually boosting up towards 4ghz like it's supposed to.
https://www.techspot.com/review/991-gta-5-pc-benchmarks/page6.html