Throughput striping Premium disk issue

Hello,

I deployed SQL server on VM (size DS14-8_v2) in Azure and to improve performance enabled disk striping to storage using premium SSD (P30 * 4 pieces). But I don't see any changes in performance. The throughput is incredible low.

For my estimation throughput should be 200 MiB/s * 4 = 800 MiB/s. Why our throughput is so low?
I'm using next guideline to setup IT environment for SQL server: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/sql/virtual-machines-windows-sql-performance

Storage Pool:

Thank you!